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

[Danial Klimkin via llvm-commits]

dklimkin wrote:

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

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