After D152389, single reduction clause constraint was removed.
This broke the OpenACC frontend test which seems to rely on specific
allowed clause ordering.
Reviewed By: vzakhari
Differential Revision: https://reviews.llvm.org/D152398
Add unit tests for functions in LLVMFrontendOpenACC. As notice in D91470 these functions were not tested
as well as the ones for OpenMP (D91643). This patch add tests for the OpenACC part.
Reviewed By: Meinersbur
Differential Revision: https://reviews.llvm.org/D91653