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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> In other words, part of the motivation of this PR is to address a general issue where Clang fails to cover the set of headers that it ought to.

I think it makes sense for Clang to handle the C Standard Library headers which can introduce `NULL`, but I'm surprised to see us adding new headers like `dbm.h`, the `sys` directory, and `unistd,h`.

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