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