ada: check Atree.Get/Set_Field_Value

Get_Field_Value and Set_Field_Value now check that the Nkind or Ekind is
correct. However, the checks are partially disabled, because they
sometimes fail.

gcc/ada/

	* atree.adb (Field_Present): New function to detect whether or not
	a given field is present in a given node, based on either the node
	kind or the entity kind as appropriate.
	(Get_Field_Value): Check that the field begin fetched exists.
	However, disable the check in the case of Scope_Depth_Value,
	because we have failures in that case. Those failures need to be
	fixed, and then the check can be enabled for all fields.
	(Set_Field_Value): Check that the field begin set exists.