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

[via cfe-commits]

jeremyd2019 wrote:

> Building libc++ with GCC is tested and supported on Linux. In MinGW environments it should probably work but it's not continuously tested. But iirc @kikairoya did test it quite recently?

I believe #141040 allows that to build/link with GCC.

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