clang-cl: Simplify handling of /arch: flag.

r213083 initially implemented /arch: support by mapping it to CPU features.
Then r241077 additionally mapped it to CPU, which made the feature flags
redundant (if harmless). This change here removes the redundant mapping to
feature flags, and rewrites test/Driver/cl-x86-flags.c to be a bit more of an
integration test that checks for preprocessor defines like AVX (like documented
on MSDN) instead of for driver flags.

To keep emitting warn_drv_unused_argument, use getLastArgNoClaim() followed by an explicit claim() if needed.

This is in preparation for adding support for /arch:AVX512(F).

No intended behavior change.

https://reviews.llvm.org/D42497

llvm-svn: 323426