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

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

Oh, also note that by default there will still be diagnostics about all incorrect placements. You'll have to opt-in to silently ignoring incorrect placements via `-Wno-dllexport-explicit-instantiation` (assuming I did things correctly).

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