[clang] Clang tooling generated visibility macros for Clang (PR #109702)

[Richard Smith via cfe-commits]

zygoloid wrote:

@fsfod Can you update the first comment to describe the overall purpose of this change? That seems pretty central to understanding whether we want this, and if so, whether this looks like the right set of annotations.

https://github.com/llvm/llvm-project/pull/109702