Disallow extension properties with backing fields

#KT-1682 Fixed

compiler/testData/codegen/box/properties/generalAccess.kt

@@ -2,7 +2,7 @@ package  As
 
 val staticProperty : String = "1"
 
-val String.staticExt  = "1"
+val String.staticExt: String get() = "1"
 
 open class A(val init: String) {
 
@@ -10,7 +10,7 @@ open class A(val init: String) {
 
     private val privateProperty : String = init
 
-    val String.ext  = "1"
+    val String.ext: String get() = "1"
 
     val Int.myInc : Int
         get() = this + 1