[llvm] [LLVM] Add option to store Parent-pointer in ilist_node_base (PR #94224)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Ouch, that's an oversight on my part - I think the correct solution probably involves adding a `const_parent_ptr_ty` to `node_options`, though I think that either requires us to pass both around (making the template specialization logic a bit messier/nastier), or to pass around the parent type directly, which is probably the better choice - the only cost is that we can't set a `void*` parent pointer, but we probably don't need that!

https://github.com/llvm/llvm-project/pull/94224