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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

Does it make sense to keep the tests for all platforms then? If we decide to restrict the testing to the platforms expected to use the Clang-provided header, then I think it should at least be staged so that there is some CI cycle where the tests are run regardless (in case the system header is problematic for use with Clang on more platforms).

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