[llvm] Fix getParent / getNodeBaseParent to return pointer to const (#94224) (PR #96053)

[Stephen Tozer via llvm-commits]

https://github.com/SLTozer approved this pull request.

> Ideally, I'd prefer `ParentPtrTy` to be `ParentTy` and used as `ParentTy*` throughout but it's a much larger change.

I just commented exactly that on the original PR - I originally passed around `ParentPtrTy` to preserve the possibility of a `void*` parent pointer since it seemed like a 0-cost decision that could possibly be useful in future, but since it now results in messier template logic I'm fine to drop it.

This patch LGTM, so if you need to merge it to unblock something downstream then go ahead; if it's not so urgent, then I'll upload a patch that fixes it as described above instead.

https://github.com/llvm/llvm-project/pull/96053