Enable warnings in llvm_rs_cc

- Fixed a bug that prevented llvm_rs_cc from emitting many warnings.
- Add support so that we can do Wno-error (needed to avoid deprecation warnings
  from breaking the build).
- Add test to verify deprecated warning works.
- Simplified slang top level to clean up handling of 32/64 bit compilation handling.

Change-Id: Ibacfa7d3d9708cb39a33b71da9621aee2718f758
18 files changed