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

[Reid Kleckner via llvm-commits]

https://github.com/rnk commented:

Broadly speaking, yeah, I think this is the right direction for building libLLVM.dll. I'm looking forward to it coming together. :)

I didn't hit approve since only early feedback was requested, and I think there's stuff to iterate on.

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