[clang] [lld] [llvm] [WIP][IR][Constants] Change the semantic of `ConstantPointerNull` to represent an actual `nullptr` instead of a zero-value pointer (PR #166667)

[Shilei Tian via cfe-commits]

shiltian wrote:

Since this PR is literally choking the browser and GitHub, I'm preparing a stack of PRs that will take us from a few NFC changes to the final semantic changes step by step. The new series will also include all the feedback we've already received on this PR.

https://github.com/llvm/llvm-project/pull/166667