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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> We could add a builtin matching the GCC `__builtin_tgmath` builtin and remove our header for at least those targets, though.

Clang would either need to change its `__GNUC__` value or the glibc header would need to add a `__has_builtin` query.
Oddly enough, `__has_builtin(__builtin_tgmath)` evaluates to zero for GCC.

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