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

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h-vetinari wrote:

> Clang would either need to change its `__GNUC__` value

Independently of the discussion here, that value should IMO be updated (https://github.com/llvm/llvm-project/issues/42162) or at least documented (https://github.com/llvm/llvm-project/issues/99804)

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