[clang-cl] Ignore /Wv and /Wv:17 flags

MSVC supports passing /Wv and /Wv:17 to ignore warnings added
since that version. Clang doesn't have a option like this - but
we can ignore this flag instead of error.

MSVC documentation: https://docs.microsoft.com/en-us/cpp/build/reference/compiler-option-warning-level

Reviewed By: hans, mstorsjo

Differential Revision: https://reviews.llvm.org/D122298