[LLVMFronted][tests] Add basic OpenMP parsing tests.

As noticed in D91470, some of the functions of LLVMFrontend, are not tested within the library itself (but indirectly by its users clang and flang). In particular, the file OMP.cpp which is generated by tablegen was not tested at all.

Add tests for the parsing helpers in OMP.cpp. These are not meant to be exhaustive tests, just to ensure that we have some basic tests for all API functions.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D91643