clang-cl: Ignore the /wd n flag for disabling a warning

Clang doesn't have a table mapping cl.exe to clang warnings.  While some
warnings like -Wsign-compare exist in both compilers, the majority do
not correspond and should usually be ignored.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188732 91177308-0d34-0410-b5e6-96231b3b80d8