[FIR/IR generator] Factor out walkableChildren and transformableChildren

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

@@ -80,10 +80,6 @@ abstract class AbstractElement<Element, Field>(
 
     abstract override val allFields: List<Field>
 
-    abstract override val walkableChildren: List<Field>
-
-    abstract override val transformableChildren: List<Field>
-
     /**
      * A custom return type of the corresponding transformer method for this element.
      */

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

@@ -51,6 +51,13 @@ abstract class AbstractField {
      */
     var useInBaseTransformerDetection = true
 
+    /**
+     * Whether a visitor should be run on this field in the generated `acceptChildren` in `transformChildren` method.
+     *
+     * Only has effect if [containsElement] is `true`.
+     */
+    var needAcceptAndTransform: Boolean = true
+
     override fun toString(): String {
         return name
     }

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

@@ -21,9 +21,15 @@ interface FieldContainer {
     val hasTransformChildrenMethod: Boolean
         get() = false
 
+    /**
+     * The fields on which to run the visitor in generated `acceptChildren` methods.
+     */
     val walkableChildren: List<AbstractField>
-        get() = emptyList()
+        get() = allFields.filter { it.containsElement && !it.withGetter && it.needAcceptAndTransform }
 
+    /**
+     * The fields on which to run the transformer in generated `transformChildren` methods.
+     */
     val transformableChildren: List<AbstractField>
-        get() = emptyList()
+        get() = walkableChildren.filter { it.isMutable || it is ListField }
 }