[FIR] Add FirBackingField

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/declarations/FirJavaField.kt

@@ -55,6 +55,7 @@ class FirJavaField @FirImplementationDetail constructor(
     override val isVal: Boolean get() = !isVar
     override val getter: FirPropertyAccessor? get() = null
     override val setter: FirPropertyAccessor? get() = null
+    override val backingField: FirBackingField? = null
 
     override val origin: FirDeclarationOrigin
         get() = FirDeclarationOrigin.Java
@@ -78,6 +79,10 @@ class FirJavaField @FirImplementationDetail constructor(
         return this
     }
 
+    override fun <D> transformBackingField(transformer: FirTransformer<D>, data: D): FirField {
+        return this
+    }
+
     override fun <D> transformOtherChildren(transformer: FirTransformer<D>, data: D): FirField {
         transformAnnotations(transformer, data)
         initializer = initializer?.transformSingle(transformer, data)

compiler/fir/java/src/org/jetbrains/kotlin/fir/java/declarations/FirJavaValueParameter.kt

@@ -79,6 +79,9 @@ class FirJavaValueParameter @FirImplementationDetail constructor(
     override val setter: FirPropertyAccessor?
         get() = null
 
+    override val backingField: FirBackingField?
+        get() = null
+
     override val controlFlowGraphReference: FirControlFlowGraphReference?
         get() = null
 
@@ -131,6 +134,10 @@ class FirJavaValueParameter @FirImplementationDetail constructor(
         return this
     }
 
+    override fun <D> transformBackingField(transformer: FirTransformer<D>, data: D): FirValueParameter {
+        return this
+    }
+
     override fun <D> transformAnnotations(transformer: FirTransformer<D>, data: D): FirValueParameter {
         return this
     }