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

[Reid Kleckner]

  LGTM

  If clang doesn't warn about a second -o overriding the first with the normal driver, I don't care about clang-cl -Fo overrides.

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