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

[Alexander Richardson via llvm-commits]

https://github.com/arichardson commented:

This generally looks good to me, just a few comments about the test diffs.

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