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

[Vassil Vassilev via cfe-commits]

vgvassilev wrote:

Unless there is more discussion or comments worth addressing I'd propose to merge this PR in the next day or two as it is a bottleneck for a few others in the pipeline.

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