Accept commas between clauses in OpenMP declare variant

Add support to the Fortran parser for the OpenMP syntax that allows a comma
after the directive name and between clauses of declare variant. The C and C++
parsers already support this syntax so only a new test is added.

gcc/fortran/ChangeLog:

	* openmp.cc (gfc_match_omp_declare_variant): Match comma after directive
	name and between clauses. Emit more useful diagnostics.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/declare-variant-2.f90: Remove error test for a comma
	after the directive name. Add tests for other invalid syntaxes (extra
	comma and invalid clause).
	* c-c++-common/gomp/adjust-args-5.c: New test.
	* gfortran.dg/gomp/adjust-args-11.f90: New test.