ada: Small housekeeping work in expansion of extension aggregates

This avoids repeatedly calling Unqualify on the same node, removes a dead
call to Generate_Finalization_Actions, a redundant setting of Assignment_OK
and reuses a local variable more consistently.  No functional changes.

gcc/ada/

	* exp_aggr.adb (Build_Record_Aggr_Code): Add new variable Ancestor_Q
	to store the result of Unqualify on Ancestor.  Remove the dead call
	to Generate_Finalization_Actions in the case of another aggregate as
	ancestor part.  Remove the redundant setting of Assignment_OK.  Use
	Init_Typ in lieu of Etype (Ancestor) more consistently.