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

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

The Driver/linker-wrapper-passes.c test failure doesn't look related, that test doesn't use stddef.h or anything else. I keep trying to rebase to make it go away but it's stubborn.

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