[clang] [Sema] -Wzero-as-null-pointer-constant: don't warn for __null (PR #69126)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

I think the changes are reasonable, but should come with a release note so users know about the change in diagnostic behavior. (We have a section in `clang/docs/ReleaseNotes.rst` for diagnostic improvements.)

Otherwise, I think the changes LGTM

https://github.com/llvm/llvm-project/pull/69126