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

[Aaron Ballman via cfe-commits]

AaronBallman 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.

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)?

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