[llvm] [WIP] Extend data layout to add non zero null value for address space. (PR #83109)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> That assumption in C has leaked into LLVM IR in a lot of places. I'd love to have them all fixed but even identifying them all is probably a multi-year effort (and preventing people from adding more is even harder).

The convertible-ness is entirely in clang's domain. We already hack around this there by emitting all nulls as addrspacecast of addrspace(0) null. This is mostly about relaxing the addrspace(0) checks in the assorted null check contexts in transforms 

https://github.com/llvm/llvm-project/pull/83109