[NFC][OpenMP][CUDA] Add test for using `-x cuda -fopenmp`

This adds a very basic test in `cuda_with_openmp.cu` that just checks whether the CUDA & OpenMP integrated headers do compile, when a CUDA file is compiled with OpenMP (CPU) enabled.
Thus this basically adds the missing test for https://reviews.llvm.org/D90415.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D105322