Add `REQUIRES` and `UNSUPPORTED` test directives

[llvm-beanz]

This adds support for interpreting REQUIRES and UNSUPPORTED directives to our testing infrastructure in a way that matches LIT.

The first of the two changes is the meaty bit of the code that actually impacts how the tests are interpreted and run. The big change is to treat dxilver as a REQUIRES line instead of making it part of the RUN line.

The second change in the set is the large mechanical change rewriting test cases.

[AppVeyorBot]

:white_check_mark: Build DirectXShaderCompiler 1.0.767 completed (commit https://github.com/microsoft/DirectXShaderCompiler/commit/e68311d0e4 by @llvm-beanz)

[llvm-beanz]

I think there is enough feedback here that I'm going to need to step back and rework things. I'll mark this as a draft and iterate a bit.

[llvm-beanz]

I'm closing this since it isn't going to merge.