[Ada] Predicates and the current instance of a subtype (AI12-0068)

[Ada] Predicates and the current instance of a subtype (AI12-0068)

gcc/ada/

	* sem_attr.adb (Analyze_Attribute, Attribute_Constrained): Issue
	a warning if the attribute prefix is a current instance
	reference within an aspect of a type or subtype.
	(Address_Checks): Replace test of Is_Object (Ent) with
	Is_Object_Reference (P) so that testing for current instances
	will be done.
	(Eval_Attribute): Add test for current instance reference, to
	ensure that we still fold array attributes when current
	instances are involved, since value prefixes are allowed for
	array attributes, and will now be excluded by
	Is_Object_Reference.
	* sem_util.ads (Is_Current_Instance_Reference_In_Type_Aspect):
	New exported query function.
	* sem_util.adb (Is_Object_Reference): Return False for the case
	where N is a current instance reference within an
	aspect_specification of a type or subtype (basically if the
	reference occurs within a predicate, invariant, or DIC aspect
	expression).
	(Is_Current_Instance_Reference_In_Type_Aspect): New function
	that tests whether a node is a reference to a current instance
	formal of a predicate, invariant, or
	Default_Initial_Condition (DIC) subprogram.