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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

With additional test coverage, the changes LGTM but I'd appreciate it if @zygoloid or @ChuanqiXu9 could validate the modules logic.

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