ada: Avoid repeated iteration over private protected components

ada: Avoid repeated iteration over private protected components

The First_Entity/Next_Entity chain includes private entities, so there
it no need to iterate starting both from First_Entity and
First_Private_Entity.

Code cleanup related to improved detection of references to
uninitialized objects; behavior is unaffected.

gcc/ada/

	* sem_util.adb
	(Check_Components): Iterate using
	First/Next_Component_Or_Discriminant.
	(Has_Preelaborable_Initialization): Avoid repeated iteration with
	calls to Check_Components with First_Entity and
	First_Private_Entity.
	(Is_Independent_Object_Entity): Tune indentation.