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

[Sean Perry via cfe-commits]

perry-ca 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.

Same for z/OS.


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