ada: Fix wrong expansion of limited extension aggregate

This happens when the ancestor part is itself an aggregate: in this case,
the tag of the extension aggregate is wrongly set to that of the ancestor.

gcc/ada/

	* exp_aggr.adb (Build_Record_Aggr_Code): In the case of an extension
	aggregate of a limited type whose ancestor part is an aggregate, do
	not skip the final code assigning the tag of the extension.