[Ada] Fix resolution of Declare_Expressions involving transient scopes

[Ada] Fix resolution of Declare_Expressions involving transient scopes

gcc/ada/

	* sem_res.adb (Resolve_Declare_Expression): Use tree traversals
	to perform name capture of local entities in the expression of
	the construct.
	* exp_util.adb (Possible_Side_Effects_In_SPARK): Do not apply to
	the prefix of an attribute reference Reduce when that prefix is
	an aggregate, because it will be expanded into a loop, and has
	no identifiable type.