ada: Fix handling of aspects CPU and Interrupt_Priority

ada: Fix handling of aspects CPU and Interrupt_Priority

When resolving aspect expression, aspects CPU and Interrupt_Priority
should be handled like the aspect Priority; in particular, all these
expressions can reference discriminants of the annotated task type.

gcc/ada/

	* sem_ch13.adb (Check_Aspect_At_End_Of_Declarations): Make
	discriminants visible when analyzing aspect Interrupt_Priority.
	(Freeze_Entity_Checks): Likewise.
	(Resolve_Aspect_Expressions): Likewise for both aspects CPU and
	Interrupt_Priority.