[llvm] Sketch of build and code changes for llvm shared library build using explicit visibility annotations (PR #96630)

[Saleem Abdulrasool via llvm-commits]

compnerd wrote:

Yeah, I think that annotating the libraries with the ABI boundaries should be good as a first step. It would allow us to split up the patch along maintainer boundaries as well hopefully and keep the size of the review more manageable.

https://github.com/llvm/llvm-project/pull/96630