[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 llvm-commits]

shiltian wrote:

Still WIP. With more code changes, there are more test failures. Fixing them one by one. :-)

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