ada: Couple of small cleanups in semantic analysis of aspects

ada: Couple of small cleanups in semantic analysis of aspects

The first cleanup is to expose a consistent interface from Sem_Ch13 for the
analysis of aspects at various points of the program.  The second cleanup is
to fix the awkward implementation of the analysis of the specification for
the aspects Stable_Properties, Designated_Storage_Model, Storage_Model_Type
and Aggregate, which are always delayed, and the incorrect placement of that
of the aspect Local_Restrictions, which is never delayed.

gcc/ada/

	* freeze.adb (Freeze_All): Call Check_Aspects_At_End_Of_Declarations
	to perform the visibility check for aspects.
	* sem_ch13.ads (Check_Aspects_At_End_Of_Declarations): Declare.
	(Check_Aspect_At_Freeze_Point): Move to...
	(Check_Aspect_At_End_Of_Declarations): Move to...
	* sem_ch13.adb 	(Check_Aspect_At_Freeze_Point): ...here.
	(Check_Aspect_At_End_Of_Declarations): ...here.
	(Analyze_Aspect_Specifications): Remove peculiar processing for
	Stable_Properties, Designated_Storage_Model, Storage_Model_Type
	and Aggregate.  Move that of Local_Restrictions around.  Reset
	Aitem at the beginning of the loop for each aspect.
	(Check_Aspects_At_End_Of_Declarations): New procedure.