Fortran/OpenMP: Improve duplicate errors

Fortran/OpenMP: Improve duplicate errors

gcc/fortran/ChangeLog:

	* openmp.c (gfc_match_dupl_check, gfc_match_dupl_memorder,
	gfc_match_dupl_atomic): New.
	(gfc_match_omp_clauses): Use them; remove duplicate
	'release'/'relaxed' clause matching; improve error dignostic
	for 'default'.

gcc/testsuite/ChangeLog:

	* gfortran.dg/goacc/asyncwait-1.f95: Update dg-error.
	* gfortran.dg/goacc/default-2.f: Update dg-error.
	* gfortran.dg/goacc/enter-exit-data.f95: Update dg-error.
	* gfortran.dg/goacc/if.f95: Update dg-error.
	* gfortran.dg/goacc/parallel-kernels-clauses.f95: Update dg-error.
	* gfortran.dg/goacc/routine-6.f90: Update dg-error.
	* gfortran.dg/goacc/sie.f95: Update dg-error.
	* gfortran.dg/goacc/update-if_present-2.f90: Update dg-error.
	* gfortran.dg/gomp/cancel-2.f90: Update dg-error.
	* gfortran.dg/gomp/declare-simd-1.f90: Update dg-error.
	* gfortran.dg/gomp/error-3.f90: Update dg-error.
	* gfortran.dg/gomp/loop-2.f90: Update dg-error.
	* gfortran.dg/gomp/masked-2.f90: Update dg-error.