clang-cl: Don't warn about overriding /MD with /MT, /Fo with another /Fo, etc.

I put in the warnings because MSVC has them, but I don't think they're very
useful.

Clang does not warn about overriding flags in general, e.g. it's perfectly
fine to have -fomit-frame-pointer followed by -fno-omit-frame-pointer.

We should focus on warning where things get confusing, such as with the
/TP and /TC options. In "clang-cl /TC a.c /TP b.cc", the user might not
realize that the /TP flag will apply to both files, and we warn about that.

Differential Revision: http://llvm-reviews.chandlerc.com/D1718