[FIR] Render backing fields by FirRenderer

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -518,6 +518,12 @@ class FirRenderer(builder: StringBuilder, private val mode: RenderMode = RenderM
         if (!mode.renderPropertyAccessors) return
         println()
         pushIndent()
+
+        if (property.hasExplicitBackingField) {
+            property.backingField?.accept(this)
+            println()
+        }
+
         property.getter?.accept(this)
         if (property.getter?.body == null) {
             println()
@@ -531,6 +537,17 @@ class FirRenderer(builder: StringBuilder, private val mode: RenderMode = RenderM
         popIndent()
     }
 
+    override fun visitBackingField(backingField: FirBackingField) {
+        print(backingField.visibility.asString() + " ")
+        print("<explicit backing field>: ")
+        backingField.returnTypeRef.accept(this)
+
+        backingField.initializer?.let {
+            print(" = ")
+            it.accept(this)
+        }
+    }
+
     override fun visitSimpleFunction(simpleFunction: FirSimpleFunction) {
         visitCallableDeclaration(simpleFunction)
         simpleFunction.body?.renderBody()