[ms] Give -Wmicrosoft-enum-forward-reference a chance to fire in clang-cl, PR32736

clang-cl sets MicrosoftCompat. In that mode, we always give enums a fixed
underlying type, and for enums with fixed underlying type we never enter the
block that tries to emit ext_ms_forward_ref_enum. Fix this by requiring an
explicit underlying type when we're skipping this diagnostic.

We had a test for this warning, but it only ran in C++98 mode. clang-cl always
enables -std=c++14, so MicrosoftCompatibiliy-cxx98.cpp is a fairly useless
test. Fold it into MicrosoftCompatibility.cpp -- that way, the test checks if
-Wmicrosoft-enum-forward-reference can fire in clang-cl builds.

https://reviews.llvm.org/D32369

llvm-svn: 301032