[Ada] Crash in class-wide pre/postconditions

[Ada] Crash in class-wide pre/postconditions

gcc/ada/

	* atree.ads (Traverse_Func_With_Parent): New generic subprogram.
	(Traverse_Proc_With_Parent): Likewise.
	* atree.adb (Parents_Stack): New table used to traverse trees
	passing the parent field of each node.
	(Internal_Traverse_With_Parent): New generic subprogram.
	(Traverse_Func_With_Parent): Likewise.
	(Traverse_Proc_With_Parent): Likewise.
	* contracts.adb (Fix_Parents): New subprogram.
	(Restore_Original_Selected_Component): Enhanced to fix the
	parent field of restored nodes.
	(Inherit_Condition): Adding assertions to check the parent field
	of inherited conditions and to ensure that the built inherited
	condition has no reference to the formals of the parent
	subprogram.
	* sem_util.ads, sem_util.adb (Check_Parents): New subprogram.