OpenMP: Add 'omp requires' to Fortran (mostly parsing)

OpenMP: Add 'omp requires' to Fortran (mostly parsing)

gcc/fortran/ChangeLog:

	* gfortran.h (enum gfc_statement): Add ST_OMP_REQUIRES.
	(enum gfc_omp_requires_kind): New.
	(enum gfc_omp_atomic_op): Add GFC_OMP_ATOMIC_ACQ_REL.
	(struct gfc_namespace): Add omp_requires and omp_target_seen.
	(gfc_omp_requires_add_clause,
	(gfc_check_omp_requires): New.
	* match.h (gfc_match_omp_requires): New.
	* module.c (enum ab_attribute, attr_bits): Add omp requires clauses.
	(mio_symbol_attribute): Read/write them.
	* openmp.c (gfc_check_omp_requires, (gfc_omp_requires_add_clause,
	gfc_match_omp_requires): New.
	(gfc_match_omp_oacc_atomic): Use requires's default mem-order.
	* parse.c (decode_omp_directive): Match requires, set omp_target_seen.
	(gfc_ascii_statement): Handle ST_OMP_REQUIRES.
	* trans-openmp.c (gfc_trans_omp_atomic): Handle GFC_OMP_ATOMIC_ACQ_REL.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/requires-1.f90: New test.
	* gfortran.dg/gomp/requires-2.f90: New test.
	* gfortran.dg/gomp/requires-3.f90: New test.
	* gfortran.dg/gomp/requires-4.f90: New test.
	* gfortran.dg/gomp/requires-5.f90: New test.
	* gfortran.dg/gomp/requires-6.f90: New test.
	* gfortran.dg/gomp/requires-7.f90: New test.
	* gfortran.dg/gomp/requires-8.f90: New test.
	* gfortran.dg/gomp/requires-9.f90: New test.