backing field migration fixes

idea/testData/quickfix/migration/backingFieldSyntax/accessor.kt

@@ -0,0 +1,7 @@
+// "Migrate backing field syntax" "true"
+
+class Foo {
+    var a: Int = 0
+        get() = 0
+        set(v) { $<caret>a = v }
+}