OpenMP/Fortran: Fix module-use renaming with declare mapper/reduction

Use the same logic as in gfc_compare_derived_types to compare the
types. Additionally, the 'declare' part only permits derived types
(per syntax) and not class - while using the mapper/reduction with
CLASS variables is possible.

gcc/fortran/ChangeLog:

	* openmp.cc (gfc_omp_udm_find, gfc_omp_udr_find): Fix
	to handle derived-type renaming via module use.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/declare-mapper-6.f90: New test.
	* gfortran.dg/gomp/declare-mapper-7.f90: New test.
	* gfortran.dg/gomp/declare-reduction-3.f90: New test.
	* gfortran.dg/gomp/declare-reduction-4.f90: New test.