[FIR generator] Use a single list of parent TypeRefs for elements

(Instead of storing the list of parent elements + the map of parent
arguments)

generators/tree-generator-common/src/org/jetbrains/kotlin/generators/tree/AbstractElement.kt

@@ -20,9 +20,7 @@ interface AbstractElement<Element, Field> : ElementOrRef<Element, Field>, FieldC
 
     val params: List<TypeVariable>
 
-    val parents: List<Element>
-
-    val parentsArguments: Map<Element, Map<TypeRef, TypeRef>>
+    val parentRefs: List<ElementOrRef<Element, Field>>
 
     val overridenFields: Map<Field, Map<Field, Boolean>>
 
@@ -30,7 +28,7 @@ interface AbstractElement<Element, Field> : ElementOrRef<Element, Field>, FieldC
         get() = false
 
     override val allParents: List<ImplementationKindOwner>
-        get() = parents
+        get() = parentRefs.map { it.element }
 
     override fun getTypeWithArguments(notNull: Boolean): String = type + generics
 }