[OPENMP50]Add restrictions for memory order clauses in atomic directive.

Added restrictions for atomic directive.
1. If atomic-clause is read then memory-order-clause must not be acq_rel or release.
2. If atomic-clause is write then memory-order-clause must not be
   acq_rel or acquire.
3. If atomic-clause is update or not present then memory-order-clause
   must not be acq_rel or acquire.