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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Today, that's no longer the case.

IBM's AIX ships a `tgmath.h` that only works with the IBM non-Clang-based XL compiler.

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