Move EH spec mismatches under -fms-compatibility

-fms-extensions is intended to enable conforming language extensions and
-fms-compatibility is intended to language rule relaxations, so a user
could plausibly compile with -fno-ms-compatibility on Windows while
still using dllexport, for example.  This exception specification
validation behavior has been handled as a warning since before
-fms-compatibility was added in 2011. I think it's just an oversight
that it hasn't been moved yet.

This will help users find conformance issues in their code such as those
found in _com_ptr_t as described in https://llvm.org/PR42842.

Reviewers: hans

Subscribers: STL_MSFT, cfe-commits

Differential Revision: https://reviews.llvm.org/D66770

llvm-svn: 370087