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

[Louis Dionne via cfe-commits]

ldionne wrote:

@ian-twilightcoder Cherry-picking is easy. Once this is merged, follow the guidelines here to get it cherry-picked: https://llvm.org/docs/GitHub.html#backporting-fixes-to-the-release-branches

Basically you only need to add a comment to this PR like `/cherry-pick COMMIT-SHA` and everything else pretty much happens automatically.

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