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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Background: it's not clear to me which C Standard Library headers Clang should or should not provide, so perhaps this change is wrong for reasons I don't understand. However, we have a number of open bug reports about our implementation of tgmath.h, and there are further changes needed for C23 (like adding `__STDC_VERSION_TGMATH_H__`), so there's maintenance costs to our providing this header. I *think* where we eventually want to get to is that Clang provides only the headers it *has* to provide, but otherwise it's on the user to supply a conforming C Standard Library to compile against.

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