[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:

I also wonder if we want to introduce `ConstantZeroPointer` instead of using `ConstantExpr(inttoptr i8 0)`, since in long term it seems like we want to get rid of `ConstantExpr`. @nikic 

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