fortran: Identify arguments by their names

fortran: Identify arguments by their names

This provides a new function to get the name of a dummy argument,
so that identifying an argument can be made using just its name
instead of a mix of name matching (for keyword actual arguments)
and argument counting (for other actual arguments).

gcc/fortran/ChangeLog:
	* interface.c (gfc_dummy_arg_get_name): New function.
	* gfortran.h (gfc_dummy_arg_get_name): Declare it.
	* trans-array.c (arg_evaluated_for_scalarization): Pass a dummy
	argument wrapper as argument instead of an actual argument
	and an index number.  Check it’s non-NULL.  Use its name
	to identify it.
	(gfc_walk_elemental_function_args): Update call to
	arg_evaluated for scalarization.  Remove argument counting.