[FIR] Merge FirPropertyFieldDeclarationSymbol into FirBackingFieldSymbol

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

@@ -53,7 +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 backingField = element("BackingField", Declaration, variable, 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/ImplementationConfigurator.kt

@@ -199,7 +199,6 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
                 withGetter = true
             }
 
-            default("backingFieldSymbol", "FirBackingFieldSymbol(symbol.callableId)")
             useTypes(backingFieldSymbolType, delegateFieldSymbolType)
         }
 
@@ -332,6 +331,10 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
             kind = OpenClass
         }
 
+        impl(backingField) {
+            kind = OpenClass
+        }
+
         impl(whenSubjectExpression) {
             default("typeRef") {
                 value = "whenRef.value.subject!!.typeRef"

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

@@ -315,7 +315,6 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
 
         property.configure {
             +symbol("FirPropertySymbol")
-            +field("backingFieldSymbol", backingFieldSymbolType)
             +field("delegateFieldSymbol", delegateFieldSymbolType, nullable = true)
             +booleanField("isLocal")
             +field("bodyResolveState", propertyBodyResolveStateType, withReplace = true)
@@ -334,11 +333,8 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
         }
 
         backingField.configure {
-            +symbol("FirPropertyFieldDeclarationSymbol")
-            +field("backingFieldSymbol", backingFieldSymbolType, nullable = true).apply {
-                withBindThis = false
-            }
-            +field("propertySymbol", firPropertySymbolType, nullable = true).apply {
+            +field("symbol", backingFieldSymbolType)
+            +field("propertySymbol", firPropertySymbolType).apply {
                 withBindThis = false
             }
             +initializer.withTransform().withReplace()