Use clang driver on MinGW where clang-cl is not usable. MSVC target
still uses clang-cl to minimize changes to existing test runners.
Differential Revision: https://reviews.llvm.org/D147432
MinGW is GCC-like and does not support clang-cl, so clean up the lit cfg
to reflect that. Also mark all Windows test cases using clang-cl as
UNSUPPORTED for now. We should probably adapt most of these test cases
to use the GCC-like clang driver, but that will be for later.
Differential Revision: https://reviews.llvm.org/D147057