ada: Spurious unreferenced warning on selected component

ada: Spurious unreferenced warning on selected component

This patch fixes an error in the compiler whereby a selected component on the
left hand side of an assignment statement may not get marked as referenced -
leading to spurious unreferenced warnings on such objects.

gcc/ada/

	* sem_util.adb (Set_Referenced_Modified): Use Original_Node to
	avoid recursive calls on expanded / internal objects such that
	source nodes get appropriately marked as referenced.