[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

Author: tru

clang/include/clang/Driver/Options.td

@@ -6530,6 +6530,7 @@ def _SLASH_sdl_ : CLIgnoredFlag<"sdl-">;
 def _SLASH_utf8 : CLIgnoredFlag<"utf-8">,
   HelpText<"Set source and runtime encoding to UTF-8 (default)">;
 def _SLASH_w : CLIgnoredJoined<"w">;
+def _SLASH_Wv_ : CLIgnoredJoined<"Wv">;
 def _SLASH_Zc___cplusplus : CLIgnoredFlag<"Zc:__cplusplus">;
 def _SLASH_Zc_auto : CLIgnoredFlag<"Zc:auto">;
 def _SLASH_Zc_forScope : CLIgnoredFlag<"Zc:forScope">;

clang/test/Driver/cl-options.c

@@ -396,6 +396,8 @@
 // RUN:    /volatile:iso \
 // RUN:    /w12345 \
 // RUN:    /wd1234 \
+// RUN:    /Wv \
+// RUN:    /Wv:17 \
 // RUN:    /Zc:__cplusplus \
 // RUN:    /Zc:auto \
 // RUN:    /Zc:forScope \