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

[Saleem Abdulrasool via cfe-commits]

compnerd wrote:

> pre-commit actions are expensive, but I think we could afford to do something here. It is not hard to set up a DLL build of an ELF DSO build that uses these annotations, and it's cheaper than a fully static build.

Having a fast incremental build for DLL-enabled LLVM/clang/lld/lldb on Windows would be very useful. It should be "cheaper" than a fully static build as you point out and would ensure that this configuration doesn't regress.

> I'd like some answer about how we plan to ensure the macros remain correct.

The default behaviour of `ids` is to assume that anything declared in the header is public and it will ensure that anything added in the headers is annotated properly to say it is exposed, making something private would be an explicit decision that the developer need to take.

> Also, can the tool itself be added to llvm-project?

I've no objection to merging it into llvm-project, I had designed it to be generic enough that it could actually be vended as a useful tool from the LLVM distribution (it is in some ways an adjunct to the `llvm-ifs` tool that @plotfi and I had worked on in the past).

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