[clang] [Clang] Allow simpler visibility annotations when targeting win32 and mingw (PR #133699)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Overall I don't mind the patch.  The special attribute here is... frustrating/concerning/etc, and I'd rather we just mark the attribute as 'invalid' in some way so that we can diagnose it instead.

https://github.com/llvm/llvm-project/pull/133699