[clang] [C99] Remove the tgmath.h header (PR #135236)

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> I wonder if we can put it into an AIX-only (plus whatever other targets still need it) include directory, though, so we only find and use it on the targets where it's necessary?

That, or, generally `#include_next` except for certain platforms (like AIX)?

https://github.com/llvm/llvm-project/pull/135236