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.

Add the ability to link with an atbin

This was ommitted from commit ac6693ca

Not caught since CI does not have beta xcode installed

Support for xcode-beta toolchain

Resolve #13

This PR introduces `link-with-atbin` which allows linking with an arbitrary atbin (such as a binary release, see https://github.com/AnarchyTools/atpm/issues/1) or any other atbin you have lying around.

`link-with` has been renamed `link-with-product` and we intend to deprecate the old syntax.

Implement configurations

Fix https://github.com/AnarchyTools/atbuild/issues/90

This implements, more or less, the scaffolding described in #36.  This commit does not actually make tools behave differently in any configuration (so this is mostly a placebo) but after this, tools can adjust their behavior.

Notable additions/departures from the original proposal include:

* Support for "custom" configurations outside the built-in set
* The addition of `test` and `bench` as built-in configurations, since 2/2 developers use them
* Internal API has new "helper" methods for common "tool questions" (should we optimize, are we testing, etc.)  Ideally, custom configurations could indicate their own values for these questions, although that's outside the scope of this patch.

Doc PR to follow.

Fix WMO

We previously used a (pretty bad) hack for WMO.  This resulted in issues like #92.

Upstream now has proper support for WMO (see generally, https://github.com/apple/swift-llbuild/pull/28, https://bugs.swift.org/browse/SR-881).

We now use the upstream feature to handle this case.  We also add -num-threads support, which upstream recently added.

Note that our implementation now only works for swift-DEVELOPMENT-SNAPSHOT-2016-05-09-a and above.

Resolve #92

This was not supported earlier, but it is now

Strip dead code on OSX

Add "magic" option to disable dead stripping

Advise users to use the plugin

Support bitcode

This currently only works for xcode-toolchain due to SR-1493

That being said, it does work, which is important for production iOS use

Update our framework atllbuild option for iOS