[OpenMP][CUDA] Fix std::complex in GPU regions

The old way worked to some degree for C++-mode but in C mode we actually
tried to introduce variants of macros (e.g., isinf). To make both modes
work reliably we get rid of those extra variants and directly use NVIDIA
intrinsics in the complex implementation. While this has to be revisited
as we add other GPU targets which want to reuse the code, it should be
fine for now.

Reviewed By: tra, JonChesterfield, yaxunl

Differential Revision: https://reviews.llvm.org/D83591