[llvm] Gisel visibility (PR #223102)

[Alexis Engelke via llvm-commits]

aengelke wrote:

I'm aware this is a draft, but nonetheless: Before removing SDAG/GISel headers, we should clarify whether building out-of-tree targets is supported with a distributed LLVM and/or the dylib (I don't have an opinion on this, just want to point this out). Generally, I think removing a component from the public API deserves an RFC, if only to raise awareness and confirm consensus.

https://github.com/llvm/llvm-project/pull/223102