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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

> 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.

I looked at the failures as well and I think they're unrelated.

This LGTM and based on the last comment from @zygoloid, I think this is ready to land (and be cherry-picked to the 19.x release).

Thank you for the fix!

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