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

[Jamie Schmeiser via cfe-commits]

jamieschmeiser wrote:

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

The headers that exist on AIX that define NULL but are not mandated by the C standard to define NULL are guarded with `#ifdef _AIX`.   See previous comment:
> For those non-mandated system headers that also define it on Aix, it is conditionally forced.



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