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

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

I guess... if you think this is what's best for AIX, 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.

What logic did you use to decide which headers should use `#idfdef _AIX`?

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