[clang] [Clang] Add explicit visibility symbol macros (PR #108276)

[Vassil Vassilev via cfe-commits]

https://github.com/vgvassilev approved this pull request.

Looks good to me, too. We can wait a couple of more days for @AaronBallman to have the final say.

https://github.com/llvm/llvm-project/pull/108276