[Ada] Fix bugs in Base_Type_Only (etc.) fields

[Ada] Fix bugs in Base_Type_Only (etc.) fields

gcc/ada/

	* gen_il-gen.adb (Put_Seinfo): Generate type
	Seinfo.Type_Only_Enum based on type
	Gen_IL.Internals.Type_Only_Enum. Automatically generating a copy
	of the type will help keep them in sync.  (Note that there are
	no Ada compiler packages imported into Gen_IL.)  Add a Type_Only
	field to Field_Descriptor, so this information is available in
	the Ada compiler (as opposed to just in the Gen_IL "compiler").
	(One_Comp): Add initialization of the Type_Only field of
	Field_Descriptor.
	* gen_il-internals.ads (Image): Image function for
	Type_Only_Enum.
	* atree.ads (Node_To_Fetch_From): New function to compute which
	node to fetch from, based on the Type_Only aspect.
	* atree.adb (Get_Field_Value): Call Node_To_Fetch_From.
	* treepr.adb (Print_Entity_Field): Call Node_To_Fetch_From.
	(Print_Node_Field): Assert.
	* sinfo-utils.adb (Walk_Sinfo_Fields,
	Walk_Sinfo_Fields_Pairwise): Asserts.