[clang] [clang][headers] Including stddef.h always redefines NULL (PR #99727)

[Louis Dionne via cfe-commits]

ldionne wrote:

@ian-twilightcoder You can create an issue if you want, but you can also say /cherry-pick <COMMIT> directly from the PR as long as you add the release milestone to it.

That being said, the cherry-pick failed so you will need to create a PR manually.

https://github.com/llvm/llvm-project/pull/99727