ada: Minor tweaks to processing of Aggregate aspect

ada: Minor tweaks to processing of Aggregate aspect

The main one is to give the error for Aggregate applied to array types from
Analyze_Aspects_At_Freeze_Point instead of Check_Aspect_At_Freeze_Point, as
for the other aspects.  The message is also changed to be more direct.

gcc/ada/

	* aspects.ads (Operational_Aspect): Alphabetize.
	* sem_ch13.ads (Analyze_Aspects_At_Freeze_Point): Fix description.
	* sem_ch13.adb (Analyze_Aspects_At_Freeze_Point) <Aggregate>: Give
	the error for array types here instead of...
	(Analyze_Aspect_Specifications) <Aggregate>: Adjust comment.
	(Check_Aspect_At_Freeze_Point) <Aggregate>: ...here.