Disallow extension properties with backing fields

#KT-1682 Fixed

compiler/testData/codegen/box/extensionProperties/inClass.kt

@@ -1,11 +1,12 @@
 class Test {
-  val Int.foo: String = "OK"
+    val Int.foo: String
+        get() = "OK"
 
-  fun test(): String {
-    return 1.foo
-  }
+    fun test(): String {
+        return 1.foo
+    }
 }
 
 fun box(): String {
-   return Test().test()
-}
+    return Test().test()
+}