openmp, Fortran: Add support using iterators with custom mappers in Fortran

openmp, Fortran: Add support using iterators with custom mappers in Fortran

gcc/fortran/

	* openmp.cc (gfc_omp_instantiate_mapper): Add argument for namespace.
	Apply namespace to new clauses.  Propagate namespace to nested
	mappers.
	(gfc_omp_instantiate_mappers): Pass namespace of clause to clauses
	generated by mappers.

libgomp/

	* testsuite/libgomp.fortran/mapper-iterators-1.f90: New test.
	* testsuite/libgomp.fortran/mapper-iterators-2.f90: New test.
	* testsuite/libgomp.fortran/mapper-iterators-3.f90: New test.
	* testsuite/libgomp.fortran/mapper-iterators-4.f90: New test.

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