clang-cl: Accept (and ignore) /Zc:__cplusplus.

See https://blogs.msdn.microsoft.com/vcblog/2018/04/09/msvc-now-correctly-reports-__cplusplus/
clang-cl already sets __cplusplus to the correct value, so we can just ignore this flag.

Also add test coverage for a few more accepted-but-ignored flags.

https://reviews.llvm.org/D45877

llvm-svn: 330427