clang-cl: ignore compile-only options in link-only invocations.

Previously we would warn about unused arguments such as /MD when linking.
Clang already has logic to ignore compile-only options, e.g. for -D and -U.
This patch extends that to include clang-cl's compile-only options too.

Also, some clang-cl options should always be ignored. Doing this earlier
means they get ignored both for compilation and link-only invocations.