This commit makes the swift build scripts use the clang that we build as part of
the build process in order to build the Swift runtime, and not the host
compiler. This change is necessary for using new clang features immediately after they are introduced.
One example for such feature is new function calling conventions, or new c-level attributes.
We want to have the option of building the standard library without
-sil-serialize-all.
For now, the build crashes in the SIL verifier due to various issues;
upcoming patches will address these.
This is part of the larger goal of building the stdlib with
-enable-resilience.
Part of https://bugs.swift.org/browse/SR-267.
This brings down StdlibUnittest build time to 90 seconds with either
a DebugAssert or a ReleaseAssert compiler.
The new library, StdlibCollectionTests, is only built when running
validation tests.
