[llvm] Initial changes for llvm shared library build using explicit visibility annotations (PR #96630)

[Thomas Fransham via llvm-commits]

fsfod wrote:

I'm not sure if LLVM_LIBRARY_VISIBILITY error on gvn is the the issue @tstellar mentioned about some attribute types are only supported in different places by some compilers and older versions of clang.

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