[Ada] Remove repeated analysis of attribute prefixes

[Ada] Remove repeated analysis of attribute prefixes

At the beginning of Analyze_Attribute routine we analyze the attribute
prefix. There is no need to repeat this analysis in branches for
individual attributes.

Code cleanup related to various Analyze/Resolve routines.

gcc/ada/

	* sem_attr.adb (Analyze_Attribute): Remove calls to Analyze for
	attributes Identity, Priority, Ref, Restriction_Set, To_Address
	and for tasking-related attributes Callable, Terminated and
	Storage_Size.