[Ada] Improve code generated for assignment of dynamic record aggregates

[Ada] Improve code generated for assignment of dynamic record aggregates

2020-06-12  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_aggr.adb: Add with and use clauses for Sem_Mech.
	(Get_Base_Object): New function to get the base object of a node.
	(In_Place_Assign_OK): Add Target_Object parameter.  Deal with a
	qualified expression on entry.  Remove short-circuit for array
	aggregates with a single "others" choice.  Do not look into the
	components of the aggregate if the parent is an allocator.
	(Check_Component): Add T_OK parameter and rewrite.
	(Safe_Component): Invoke Check_Component with T_OK set to False.
	(Convert_To_Assignments): Try to use an in-place assignment for
	any target; for that, call Get_Base_Object on the target and pass
	the result to In_Place_Assign_OK.
	(Expand_Array_Aggregate): Use Parent_Kind and Parent_Node more
	consistently.  For an assignment, call Get_Base_Object on the
	target and pass the result to In_Place_Assign_OK.