Improve softFail support

The way we work around SR-1636 is we add a C language target with atbuild's new C project syntax.

This C target can access `environ` and the result follows.

I think this bootstraps OK, we'll see what CI says

See also: https://bugs.swift.org/browse/SR-1636, https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160523/019350.html

Support Xcode 8 GM

* Drop test coverage for Xcode 7 since it's likely not installed
* We now pull paths from xcode-select when we don't specify a toolchain which is saner than using hardcoded paths
* We now more robustly test if a given toolchain is from xcode 7 or not rather than rely on the filename
* Bootstrap for Xcode 8 installed to Xcode.app

Deployment target

Swift preview 6

Fixes from SP3 commits for relative path calculation

* We can't use system anymore in preview 3.  This introduces a lot of issues around envrionment variables, which can't be listed in swift :-(.  So we no longer inherit environment variables, we only set the ones we set.
    * We do pass on PWD and PATH, because otherwise that would be terrible
* Toolchain is now a part of platform config instead of something we pass around by hand.
* We now detect xcode 7 / 8 depending on whether we're using a toolchain installed to Xcode.app or Xcode-beta.app.  That's still not right, but fuck it.

Add support for C language to atllbuild

This PR lets you mix .swift, .h, and .c files all in the same atllbuild task.  It works a lot like Xcode's behavior, if you've used that.

\# Rationale

I feel the need to defend this feature, since I have been previously on the record as saying "the entire value is debatable" (https://www.mail-archive.com/swift-evolution@swift.org/msg01829.html).

There are 5 cases where I think it makes sense to add a little C to your Swift project:

* To use the odd feature Swift doesn't support.  Recently, I needed to call a variadic C function; Swift cannot call them, C is our only hope
* To work around a Swift compiler bug.  Several of my projects have this case.
* To repackage an existing Xcode project where somebody used C in it.  I have not investigated and don't want to investigate whether that somebody was sane or insane, but we should at least be able to build their project.
* To include headers from a system C library.  SwiftPM tries to solve this with module maps, however it doesn't hide the implementation details https://bugs.swift.org/browse/SR-655.  This feature can actually hide them with a few different methods discussed below, which is a clear win.
* To write Swift bindings for a C library.  This generally involves a little C glue code (such as using a header or something), and for reasons that will become clear, using our C support is better than previous approaches at that problem.

\# Rationale-NOT:

Additional rationale:

* SwiftPM will probably add this eventually
* per #113, we should be a superset of their functionality

I would like to be very clear about my goals:

* This is really only designed for the case of "need a little C in your Swift project", not anything larger
* This is not a replacement for e.g. GNU Make or a general-purpose C buildsystem, nor will it become one
* The preferred mechanism for building a real C library is shelling out to your real C buildsystem
* Nobody should be repackaging their established C libraries as atllbuild tasks.  atllbuild is designed to build Swift projects, not C projects.

\# Design

* You can now specify `.c` and `.h` files in the sources for atllbuild tasks
    * Also `**.c` and `**.h` just like `**.swift`
    * Like `.swift`, no files are scanned by default, everything is explicit
* Adding `.c` files causes them to be compiled and linked into the atllbuild task just like swift files
* Adding `.h` files exposes declarations to Swift.  It works much like a bridging header; put stuff in header files and then Swift code will see it.
    * Your `.h`s can import other `.h`s (from the system, or anywhere else) and you otherwise have access to the complete C preprocessor
* New atllbuild setting `c-compile-options` specifies compile options for C files.  `compile-options` is ignored for C.
* C files work as you'd expect, including support for things like configurations, optimization, atbuild preprocessor macros, etc.

\# Linking

The standard `link-options` sets link options for both C and Swift; since they are linked into the same library there is no individual control.  So if you want to link your C (and Swift) code against curl, you could say `:link-options ["-lcurl"]` for example.

The problem with this approach is that everybody who depends on you might also need `-lcurl`.  Traditionally we've solved this with overlays that we expose to callers.

SwiftPM avoids this problem by requiring everyone to create e.g. `CCurl` everywhere: https://github.com/apple/swift-package-manager/blob/master/Documentation/SystemModules.md

And in fact people do: https://github.com/IBM-Swift/CCurl

The problem is now you have to import `CCurl` everywhere (even in files that don't directly use it).  See generally, https://bugs.swift.org/browse/SR-655, https://gist.github.com/briancroom/5d0f1b966fa9ef0ae4950e97f9d76f77

Here is the cool part though.  This PR adds a new option `:module-map-link ["curl"]`.  That will inject a link directive into both the module map we use at buildtime and the one we export e.g. into an atbin.

Emitting that link directive has the effect of injecting `:link-options ["-lcurl"]`.  However, it will *also* inject that link option into any Swift module that imports this one.  The result is that downstream no longer needs to add `:link-options ["-lcurl"]` anymore.

Additonally, since we achieve this in a single module, there is no `CCurl` to import anymore.  The details of linking to the C library are more effectively hidden.

For these reasons, I believe using the C support in this PR is way more effective for writing bindings than any other solution.

\# Known issues

* Using `.h` in `sources` requires a synthesized module map
* Using `.c` in `sources` is not supported for bitcode
* Using `module-map-link` requires the module map to be distributed for the link to take effect on downstream; we recommend `packageatbin` for packing build products
* Currently, swift functions are not "visible" to C code (like they are visible to ObjC from Xcode) although presumably if you had a function, knew its calling convention, and knew its c-name, you could totally call it from C.

Add `atbuild bootstrap` task to update all bootstraps

* bugfix related to automatically importing an external dependency with SSH-based URL
* link-with => link-with-products

Add actual effects to the configurations

Remove atllbuild option swiftc-path

We can't enable WMO for Release on Linux yet, see https://github.com/AnarchyTools/atbuild/issues/110

This extends #104 by adding actual effects to the configurations rather than have them be no-ops.

Effects include:

* debug instrumentation (new in this PR), for emitting `-g` (see #73 for an obvious extension)
* optimization control / WMO control
* compression level (faster debug atbins)
* test instrumentation (`-enable-testing`)
* `#if ATBUILD_RELEASE` etc. from Swift code

There are some deprecations associated with this PR:

* `whole-module-optimization` atllbuild option is now deprecated; use `--configuration release` instead.  There currently is no plan to control these separately, use `--configuration plain` + `:compileOptions ["-O"]` to get optimization without WMO.  Or open a bug to complain about this change.
* `magic` atllbuild option is now deprecated; to opt out of magic use `--configuration none` instead.

Doc PR to follow.

In addition, CI is now updated to produce release (optimized) builds for atbuild, which significantly optimizes atbuild performance.

This option was deprecated in atbuild 0.9.0.  Use `--toolchain` on the CLI instead.

Users should have had enough time to migrate at this point.