fortran: simplify elemental arguments walking

This adds two functions working with the wrapper struct gfc_dummy_arg
and makes usage of them to simplify a bit the walking of elemental
procedure arguments for scalarization.  As information about dummy arguments
can be obtained from the actual argument through the just-introduced
associated_dummy field, there is no need to carry around the procedure
interface and walk dummy arguments manually together with actual arguments.

gcc/fortran/ChangeLog:
	* interface.c (gfc_dummy_arg_get_typespec,
	gfc_dummy_arg_is_optional): New functions.
	* gfortran.h (gfc_dummy_arg_get_typespec,
	gfc_dummy_arg_is_optional): Declare them.
	* trans.h (gfc_ss_info::dummy_arg): Use the wrapper type
	as declaration type.
	* trans-array.c (gfc_scalar_elemental_arg_saved_as_reference):
	use gfc_dummy_arg_get_typespec function to get the type.
	(gfc_walk_elemental_function_args): Remove proc_ifc argument.
	Get info about the dummy arg using the associated_dummy field.
	* trans-array.h (gfc_walk_elemental_function_args): Update declaration.
	* trans-intrinsic.c (gfc_walk_intrinsic_function):
	Update call to gfc_walk_elemental_function_args.
	* trans-stmt.c (gfc_trans_call): Ditto.
	(get_proc_ifc_for_call): Remove.