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

[Richard Smith via cfe-commits]

zygoloid wrote:

> > For some reason, WG21 has trampled over that meaning of "freestanding" and we no longer have such a separation for C++, but it still exists for C
> 
> See #132232.

Sigh. So we can't use "freestanding" as the line between compiler and standard library anywhere any more, not even in C.

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