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

[Aaron Ballman via cfe-commits]

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

The changes LGTM with some minor changes to the comments. I did have a question, but I don't think it holds up landing the changes.

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