Turn on -Wmicrosoft-enum-forward-reference by default.

clang-cl already errs or warns on everything that cl
warns on in /permissive- mode, except for enum foward
declarations (and ATL attributes).
So warn on enum forward declarations by default.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302190 91177308-0d34-0410-b5e6-96231b3b80d8