[clang] Don't emit warn_cxx_ms_struct when MSBitfields is enabled globally

This diagnostic (which defaults to an error, added in
95833f33) was intended to clearly
point out cases where the C++ ABI won't match the Microsoft C++ ABI,
for cases when this is enabled via a pragma over a region of code.

The MSVC compatible struct layout feature can also be enabled via a
compiler option (-mms-bitfields). If enabled that way, one essentially
can't compile any C++ code unless also building with
-Wno-incompatible-ms-struct (which GCC doesn't support, and projects
developed with GCC aren't setting).

For the MinGW target, it's expected that the C++ ABI won't match
the MSVC one, if this option is used for getting the struct
layout to match MSVC.

Differential Revision: https://reviews.llvm.org/D81794