openmp: Add support for using custom mappers with iterators (C, C++)

openmp: Add support for using custom mappers with iterators (C, C++)

gcc/c-family/

	* c-omp.cc (omp_instantiate_mapper): Apply iterator to new clauses
	generated from mapper.

gcc/c/

	* c-parser.cc (c_parser_omp_clause_map): Apply iterator to push and
	pop mapper clauses.

gcc/cp/

	* parser.cc (cp_parser_omp_clause_map): Apply iterator to push and
	pop mapper clauses.
	* semantics.cc (cxx_omp_map_array_section): Allow array types for
	base type of array sections.

libgomp/

	* testsuite/libgomp.c-c++-common/mapper-iterators-1.c: New test.
	* testsuite/libgomp.c-c++-common/mapper-iterators-2.c: New test.
	* testsuite/libgomp.c-c++-common/mapper-iterators-3.c: New test.

Co-authored-by: Andrew Stubbs <ams@baylibre.com>