Disallow extension properties with backing fields

#KT-1682 Fixed

compiler/testData/codegen/box/properties/kt1892.kt

@@ -1,5 +1,5 @@
-val Int.ext : () -> Int = { 5 }
-val Long.ext : Long = 4.ext().toLong()  //(c.kt:4)
-val y : Long = 10.toLong().ext
+val Int.ext: () -> Int get() = { 5 }
+val Long.ext: Long get() = 4.ext().toLong()  //(c.kt:4)
+val y: Long get() = 10L.ext
 
-fun box() : String = if (y == 5.toLong()) "OK" else "fail"
+fun box(): String = if (y == 5L) "OK" else "fail: $y"