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

[Saleem Abdulrasool via cfe-commits]

compnerd wrote:

> Looking at Format.h (the only file I'm interested in), there feels to me to be things in the ABI that I wouldn't expect to be called externally, I don't know enough about what's involved in creating a DLL ABI but I wouldn't want some of the clang-format functions to be in a situation where we could change their parameters because it would be ABI breaking. @owenca thoughts?

Note that ABI stability is well outside of the scope of this project. The annotations here are to enable the use of a single clang DLL that would allow us to share that across all the tools to shrink the distribution size as well as gain some page sharing between different tools when running. This might also help us restore some additional backends to the prebuilt Windows toolchain distributions from llvm.org.

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