ada: Improve accessibility check generation

ada: Improve accessibility check generation

Improve accessibility check generation by more precisely identifying cases in
which an Original_Node call is needed.

Instead of grabbing the Original_Node of a prefix in all cases (since this
can cause issues where unanalyzed instance names get referenced) we only
obtain the original node when said prefix comes as a result of expanding
function calls.

gcc/ada/

	* sem_util.adb
	(Accessibility_Level): Modify indexed and selected components case
	by reducing the scope where Original_Node gets used.