ada: Support Constant_Reference in Iterable aspect

ada: Support Constant_Reference in Iterable aspect

A Constant_Reference function returning an anonymous access-to-constant
type can be supplied instead of the Element primitive to allow
iteration over elements of a container.

gcc/ada/ChangeLog:

	* exp_ch5.adb (Expand_Formal_Container_Element_Loop): Use
	Constant_Reference instead of Element if necessary.
	* sem_ch13.adb (Resolve_Iterable_Operation): Check Constant_Reference.
	(Validate_Iterable_Aspect): Handle Constant_Reference if supplied.
	* sem_ch5.adb (Analyze_Loop_Parameter_Specification): Iteration over
	elements of a container is allowed if Constant_Reference is supplied.
	* sem_util.adb (Get_Iterable_Type_Primitive): Fix assertion.
	* snames.ads-tmpl (Name_Constant_Reference): New name.