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

[Vassil Vassilev via llvm-commits]

vgvassilev wrote:

@fsfod, @compnerd, @rnk, what is the preferred way to move forward here? If we want atomic history we should probably land the whole pull request. It is not clear to me if we can chunk it at a reasonable level...

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