[llvm] [RFC][DataLayout] Add null pointer value infrastructure (PR #183207)

[Shilei Tian via llvm-commits]

shiltian wrote:

> I think this PR should only deal with the representation part, dereferencability of `null` should not be changed here (and I _think_ it is not changing yet, but I could be wrong).

Yes, this PR doesn't change anything regarding null pointer. The "work" I referred before is the final semantic change of `ConstantPointerNull`.

https://github.com/llvm/llvm-project/pull/183207