[LLVM][OpenMP] Implement getLeafOrCompositeConstructs

This function will break up a construct into constituent leaf and
composite constructs, e.g. if OMPD_c_d_e and OMPD_d_e are composite
constructs, then OMPD_a_b_c_d_e will be broken up into the list
{OMPD_a, OMPD_b, OMPD_c_d_e}.