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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Thank you for pointing that out! So IBM still needs Clang to vend this for now, I take it? Is IBM planning to update their tgmath.h so that it works with the Clang-based XL compiler (so we can eventually remove Clang's)?

The road to being able to retain the functionality with Clang while also removing the header from Clang is a long one. Enabling use of the system `tgmath.h` with Clang is "new functionality" that would only be made available for newer releases of AIX. The compiler is typically used on older releases that receive bug fixes but no new functionality.

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