ada: Improve check of attribute reference

ada: Improve check of attribute reference

Before this patch, the front end failed to catch many illegal uses
of access attributes of task types.

This patch makes referring to the access attributes of a task type
raise an error, except in the current instance case defined in
clause 8.6 of the reference manual.

gcc/ada/

	* sem_attr.adb: sem_attr.adb (Analyze_Access_Attribute): Tighten
	validity check for task types.