sem_ch12.adb (Save_Entity_Descendant): Use syntactic field names on known node types...

sem_ch12.adb (Save_Entity_Descendant): Use syntactic field names on known node types, rather than untyped fields.

	* sem_ch12.adb (Save_Entity_Descendant): Use syntactic field names
	on known node types, rather than untyped fields. Further cleanups.

	* sem_ch12.adb:
	(Save_Entity_Descendant): Minor comment update.
	(Copy_Generic_Node): Deal with incorrect reference to Associated_Node
	 of an N_Attribute_Reference node. As per note below, this does not
	eliminate need for Associated_Node in attribute ref nodes.
	(Associated_Node): Documentation explicitly mentions attribute
	reference nodes, since this field is used in such nodes.

	* sem_ch12.adb (Associated_Node): Minor documentation cleanup.

From-SVN: r47954