Disallow extension properties with backing fields

#KT-1682 Fixed

compiler/testData/codegen/box/labels/propertyAccessorInnerExtensionFun.kt

@@ -7,7 +7,8 @@ val Int.getter: Int
     }
 
 
-var Int.setter: Int = 1
+var Int.setter: Int
+    get() = 1
     set(i: Int) {
         val extFun = { Int.() ->
             this@setter
@@ -24,4 +25,4 @@ fun box(): String {
     if (i.setter != 1) return "setter failed"
 
     return "OK"
-}
+}