[clang] [llvm] [RFC][NVPTX] Update data layout string to include null pointer value spec (PR #195178)

[Shilei Tian via cfe-commits]

shiltian wrote:

I'm going to close this PR at this moment, since the infrastructure is not ready so to update them right now will cause some inconsistency, but please do let me know what ASs have non-zero null pointer value such that I can update them locally to test my changes.

FYI, https://github.com/llvm/llvm-project/pull/183215 this PR stack shows where we are heading.

https://github.com/llvm/llvm-project/pull/195178