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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > These changes should come with a release note so users know about the fix.
> 
> Does it need a release note? It was introduced in 19 with #90676, and this will fix it in 19 too.

Ah nope, no need for a release note then!

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