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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

What I am taking away from this conversation is: 1) We need to keep tgmath.h, so this PR can be closed. 2) We should consider implementing `__builtin_tgmath`, 3) We should consider bumping the `__GNUC__` version we report. I think all of those are sensible, though (2) seems more straightforward than (3).

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