OpenMP: Fortran "!$omp declare mapper" parser support

OpenMP: Fortran "!$omp declare mapper" parser support

gcc/fortran/ChangeLog:

	* dump-parse-tree.cc (show_attr): Show omp_udm_artificial_var flag.
	(show_omp_namelist): Support OMP_MAP_UNSET.
	* gfortran.h (enum gfc_statement): Add ST_OMP_DECLARE_MAPPER.
	(symbol_attribute): Add omp_udm_artificial_var attribute.
	(enum gfc_omp_map_op): Add OMP_MAP_UNSET.
	(gfc_omp_namelist_udm): New struct.
	(gfc_omp_namelist): Add udm pointer to u2 union.
	(gfc_symtree): Add omp_udm pointer.
	(gfc_namespace): Add omp_udm_root symtree and omp_udm_ns flag.
	(gfc_free_omp_udm, gfc_omp_udm_find, gfc_find_omp_udm,
	gfc_resolve_omp_udms): Add prototypes.
	* match.h (gfc_match_omp_declare_mapper): Add prototype.
	* match.cc (gfc_free_omp_namelist): Update for declare mapper's udm.
	* openmp.cc (gfc_omp_directives): Uncomment 'declare mapper'.
	(gfc_free_omp_udm, gfc_find_omp_udm, gfc_omp_udm_find,
	gfc_match_omp_declare_mapper, gfc_resolve_omp_udm,
	gfc_resolve_omp_udms): New.
	(gfc_match_omp_clauses): Take argument for the default map-type
	modifier; add support for the 'mapper' modifier.
	(resolve_omp_clauses): Update for declare-mapper map clauses.
	* parse.cc (decode_omp_directive): Add declare mapper support.
	(case_omp_decl): Add ST_OMP_DECLARE_MAPPER case.
	(gfc_ascii_statement): Add ST_OMP_DECLARE_MAPPER case.
	* resolve.cc (resolve_types): Call gfc_resolve_omp_udms.
	* symbol.cc (free_omp_udm_tree): New function.
	(gfc_free_namespace): Call it.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/declare-mapper-1.f90: New test.
	* gfortran.dg/gomp/declare-mapper-2.f90: New test.

Co-Authored-By: Tobias Burnus <tburnus@baylibre.com>