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

[Ian Anderson via cfe-commits]

ian-twilightcoder 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 https://github.com/llvm/llvm-project/pull/90676, and this will fix it in 19 too.

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