[IR generator] Simplify configuration of visitor parents

Don't specify a visitor parent manually for each IR node,
use an algorithm instead.

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

@@ -56,10 +56,16 @@ abstract class AbstractElement<Element, Field>(
      */
     abstract val visitorParameterName: String
 
+    /**
+     * @see parentInVisitor
+     */
+    var customParentInVisitor: Element? = null
+
     /**
      * The default element to visit if the method for visiting this element is not overridden.
      */
-    abstract val parentInVisitor: Element?
+    open val parentInVisitor: Element?
+        get() = customParentInVisitor ?: elementParents.singleOrNull()?.element?.takeIf { !it.isRootElement }
 
     override val allParents: List<Element>
         get() = elementParents.map { it.element }
@@ -92,7 +98,7 @@ abstract class AbstractElement<Element, Field>(
     /**
      * The return type of the corresponding transformer method for this element.
      *
-     * By default, computed using [org.jetbrains.kotlin.generators.tree.detectBaseTransformerTypes], but can be customizaed via
+     * By default, computed using [org.jetbrains.kotlin.generators.tree.detectBaseTransformerTypes], but can be customized via
      * [transformerReturnType]
      */
     val transformerClass: Element

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

@@ -11,7 +11,7 @@ fun <Element : AbstractElement<Element, *>> Element.traverseParents(block: (Elem
 }
 
 /**
- * For each tree element, sets its [AbstractElement.baseTransformerType] to one of it parents if that parent type is used at least once as
+ * For each tree element, sets its [AbstractElement.baseTransformerType] to one of its parents if that parent type is used at least once as
  * a type of a field, except when that field is explicitly opted out of it via
  * [AbstractField.useInBaseTransformerDetection].
  */