ada: Add guard before querying the type for its interfaces

Fix crash on illegal code, when routine Iface_Present_In_Ancestor is
called on the predefined String type and attempts to examine the list of
interfaces.

gcc/ada/

	* sem_type.adb (Iface_Present_In_Ancestor): Only look at the list of
	interfaces for types that allow it. The guard is a high-level equivalent
	of the entity kinds listed in the preconditon of the Interfaces query.

gcc/ada/sem_type.adb

@@ -2578,7 +2578,9 @@ package body Sem_Type is
          end if;
 
          loop
-            if Present (Interfaces (E)) then
+            if Is_Record_Type (E)
+              and then Present (Interfaces (E))
+            then
                Elmt := First_Elmt (Interfaces (E));
                while Present (Elmt) loop
                   AI := Node (Elmt);