[FIR] Add FirBackingField

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/FirTreeBuilder.kt

@@ -53,6 +53,7 @@ object FirTreeBuilder : AbstractFirTreeBuilder() {
     val contractDescriptionOwner = sealedElement("ContractDescriptionOwner", Declaration)
     val simpleFunction = element("SimpleFunction", Declaration, function, contractDescriptionOwner, typeParametersOwner)
     val propertyAccessor = element("PropertyAccessor", Declaration, function, contractDescriptionOwner, typeParametersOwner)
+    val backingField = element("BackingField", Declaration, typedDeclaration, typeParametersOwner, statement)
     val constructor = element("Constructor", Declaration, function, typeParameterRefsOwner)
     val file = element("File", Declaration, annotatedDeclaration)
     val packageDirective = element("PackageDirective", Other)

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -333,6 +333,20 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
             +typeParameters
         }
 
+        backingField.configure {
+            +symbol("FirPropertyFieldDeclarationSymbol")
+            +field("backingFieldSymbol", backingFieldSymbolType, nullable = true).apply {
+                withBindThis = false
+            }
+            +field("propertySymbol", firPropertySymbolType, nullable = true).apply {
+                withBindThis = false
+            }
+            +initializer.withTransform().withReplace()
+            +annotations
+            +typeParameters
+            +status.withTransform()
+        }
+
         declarationStatus.configure {
             +visibility
             +modality
@@ -377,6 +391,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
             generateBooleanFields("var", "val")
             +field("getter", propertyAccessor, nullable = true).withTransform()
             +field("setter", propertyAccessor, nullable = true).withTransform()
+            +field("backingField", backingField, nullable = true).withTransform()
             +annotations
             needTransformOtherChildren()
         }