openmp: Reject #pragma omp atomic update, [PR101297]

openmp: Reject #pragma omp atomic update, [PR101297]

I've noticed that we allow a trailing comma on OpenMP atomic construct
if there is at least one clause.  Commas should be only allowed to
separate the clauses (or in OpenMP 5.1 to separate directive name
from the clauses).

2021-07-02  Jakub Jelinek  <jakub@redhat.com>

	PR c/101297
	* c-parser.c (c_parser_omp_atomic): Consume comma only if it
	appears before a CPP_NAME.

	* parser.c (cp_parser_omp_atomic): Consume comma only if it
	appears before a CPP_NAME.

	* c-c++-common/gomp/atomic-24.c: New test.