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

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

> > @AaronBallman Should this be added to the 19.x milestone and cherry-picked?
> 
> I think it should be

Nuts, should've merged it yesterday.

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