ada: Fix classification of SPARK Boolean aspects

ada: Fix classification of SPARK Boolean aspects

The implementation of User_Aspect_Definition uses subtype
Boolean_Aspects to decide which existing aspects can be used to define
old aspects. This subtype didn't include many of the SPARK aspects,
notably the Always_Terminates.

gcc/ada/

	* aspects.ads (Aspect_Id, Boolean_Aspect): Change categorization
	of Boolean-valued SPARK aspects.
	* sem_ch13.adb (Analyze_Aspect_Specification): Adapt CASE
	statements to new classification of Boolean-valued SPARK
	aspects.