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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

One thing I don't understand about this PR is why we need Compiler.h in Clang -- wouldn't the LLVM definitions in their Compiler.h work for Clang as well? (This would probably be worth explaining in the patch summary.)

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