OpenMP/Fortran: Improve 'declare reduction' diagnostic

gcc/fortran/ChangeLog:

	* openmp.cc (match_udr_expr, gfc_match_omp_declare_reduction,
	gfc_resolve_omp_udr): Improve diagnostic.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/declare-reduction-2.f90: Update dg-error.
	* gfortran.dg/gomp/udr1.f90: Likewise.
	* gfortran.dg/gomp/udr2.f90: Likewise.
	* gfortran.dg/gomp/udr3.f90: Likewise.
	* gfortran.dg/gomp/udr4.f90: Likewise.
	* gfortran.dg/gomp/declare-reduction-5.f90: New test.