ada: Fix for Default_Component_Value with declare expressions

ada: Fix for Default_Component_Value with declare expressions

When the expression of aspect Default_Component_Value includes a declare
expression with current type instance, we attempted to recursively froze
that type, which itself caused an infinite recursion, because we didn't
properly manage the scope of declare expression.

This patch fixes both the detection of the current type instance and
analysis of the expression that caused recursive freezing.

gcc/ada/

	* sem_attr.adb (In_Aspect_Specification): Use the standard
	condition that works correctly with declare expressions.
	* sem_ch13.adb (Analyze_Aspects_At_Freeze_Point): Replace
	ordinary analysis with preanalysis of spec expressions.