openmp, fortran: Move udm field of gfc_omp_namelist into a new union

openmp, fortran: Move udm field of gfc_omp_namelist into a new union

This patch moves u2.udm into u3.udm.

This is necessary to avoid clashes when mappers are used together with
iterators, which uses u2.ns.

gcc/fortran/

	* gfortran.h (struct gfc_omp_namelist): Move udm field into a new
	union u3.
	* match.cc (gfc_free_omp_namelist): Change references to u2.udm to
	u3.udm.
	* module.cc (load_omp_udms): Likewise.
	(write_omp_udm): Likewise.
	* openmp.cc (gfc_match_motion_var_list): Likewise.
	(gfc_match_omp_clauses): Likewise.
	(resolve_omp_clauses): Likewise.
	(gfc_omp_instantiate_mapper): Likewise.
	* trans-openmp.cc (gfc_trans_omp_clauses): Likewise.
	(gfc_find_nested_mappers): Likewise.