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

[Thomas Fransham via cfe-commits]

fsfod wrote:

> 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.)

We could if we didn't need the visibility macros in different states of dllexport and dllimport in the same translation unit for like LLVM symbol to be import and a Clangsymbol to be exported.

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