[FIR/IR generator] Share visitor-related properties between FIR & IR

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

@@ -40,6 +40,10 @@ abstract class AbstractElement<Element, Field> : ElementOrRef<Element, Field>, F
 
     abstract override val allFields: List<Field>
 
+    abstract override val walkableChildren: List<Field>
+
+    abstract override val transformableChildren: List<Field>
+
     final override fun get(fieldName: String): Field? {
         return allFields.firstOrNull { it.name == fieldName }
     }

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

@@ -8,4 +8,22 @@ package org.jetbrains.kotlin.generators.tree
 interface FieldContainer {
     val allFields: List<AbstractField>
     operator fun get(fieldName: String): AbstractField?
+
+    val hasAcceptMethod: Boolean
+        get() = false
+
+    val hasTransformMethod: Boolean
+        get() = false
+
+    val hasAcceptChildrenMethod: Boolean
+        get() = false
+
+    val hasTransformChildrenMethod: Boolean
+        get() = false
+
+    val walkableChildren: List<AbstractField>
+        get() = emptyList()
+
+    val transformableChildren: List<AbstractField>
+        get() = emptyList()
 }