[clang] Ensure proper NULL macro definition for system include files. (PR #149176)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> I'm not going to argue very hard about it, but I don't really want to entangle every other operating system in this. The current situation is mostly stable.

@efriedma-quic, are you saying that you have a strong preference to not handle `locale.h` (for example) for non-AIX environments? `locale.h` is expected to define `NULL` via some mechanism on every platform.

https://github.com/llvm/llvm-project/pull/149176