[FIR/IR generator] Refactor element parents

- Move `elementParents` and `otherParents` properties
  to `AbstractElement`
- Use `elementParents` instead of `parentRefs` where needed

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/model/Model.kt

@@ -22,11 +22,8 @@ class Element(
     override val fields: MutableSet<Field>,
     val additionalFactoryMethodParameters: MutableList<Field>,
 ) : AbstractElement<Element, Field>() {
-    var elementParents: List<ElementRef> = emptyList()
-    var otherParents: List<ClassRef<*>> = emptyList()
-
-    override val parentRefs: List<ElementOrRef>
-        get() = elementParents
+    override var elementParents: List<ElementRef> = emptyList()
+    override var otherParents: List<ClassRef<*>> = emptyList()
 
     override val allFields: List<Field>
         get() = fields.toList()

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/print/Elements.kt

@@ -40,7 +40,7 @@ fun printElements(generationPath: File, model: Model) = sequence {
             )
             addTypeVariables(element.params.map { it.toPoet() })
 
-            val (classes, interfaces) = (element.elementParents + element.otherParents).partition { it.typeKind == TypeKind.Class }
+            val (classes, interfaces) = element.parentRefs.partition { it.typeKind == TypeKind.Class }
             classes.singleOrNull()?.let {
                 superclass(it.toPoet())
             }
@@ -77,7 +77,7 @@ fun printElements(generationPath: File, model: Model) = sequence {
                 }.build())
             }
 
-            val isRootElement = element.elementParents.isEmpty()
+            val isRootElement = element.isRootElement
             val acceptMethodName = "accept"
             val transformMethodName = "transform"
             if (element.accept) {

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/print/Types.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.ir.generator.model.Element.Companion.elementName2typ
 import org.jetbrains.kotlin.ir.generator.util.*
 import org.jetbrains.kotlin.types.Variance
 import  org.jetbrains.kotlin.generators.tree.ElementRef as GenericElementRef
+import  org.jetbrains.kotlin.generators.tree.ElementOrRef as GenericElementOrRef
 
 fun Element.toPoet() = ClassName(packageName, typeName)
 fun Element.toPoetSelfParameterized() = toPoet().parameterizedByIfAny(poetTypeVariables)
@@ -68,7 +69,6 @@ fun TypeVariable.toPoet() = TypeVariableName(
 
 val ClassOrElementRef.typeKind: TypeKind
     get() = when (this) {
-        is GenericElementRef<*, *> -> element.kind!!.typeKind
+        is GenericElementOrRef<*, *> -> element.kind!!.typeKind
         is ClassRef<*> -> kind
-        else -> error("Unexpected type: $this")
     }