Disallow extension properties with backing fields

#KT-1682 Fixed

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

@@ -0,0 +1,21 @@
+class A {
+    var result = "Fail"
+
+    private var Int.foo: String
+        get() = result
+        private set(value) {
+            result = value
+        }
+
+    fun run(): String {
+        class O {
+            fun run() {
+                42.foo = "OK"
+            }
+        }
+        O().run()
+        return (-42).foo
+    }
+}
+
+fun box() = A().run()