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

[Arseny Kapoulkine via cfe-commits]

zeux wrote:

Let me know if I need to do anything else here -- I don't have commit rights so someone else will need to press the merge button :)

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