clang/HIP: Add new header test for math IR gen

The current header testing is pretty thin. This is in
preparation for a series of patches to replace many
builtin implementations.

I did try to stress everything in this header, but skipped
a few things. Mostly I didn't understand why we have
various language version checks which skip defining some
things. It doesn't seem right to have any of these if guards
on __cplusplus,  __HIPCC_RTC__, and __OPENMP_AMDGCN__.