Disallow extension properties with backing fields

#KT-1682 Fixed

compiler/testData/diagnostics/tests/extensions/kt819ExtensionProperties.kt

@@ -1,4 +1,3 @@
-
 //KT-819 Redeclaration error for extension properties with the same name and different receivers
 
 import java.io.*
@@ -15,16 +14,20 @@ open class A() {
     open fun String.foo() {}
     open fun Int.foo() {}
 
-    open val String.foo = 0
-    open val Int.foo = 1
+    open val String.foo: Int
+        get() = 0
+    open val Int.foo: Int
+        get() = 1
 }
 
 class B() : A() {
     override fun String.foo() {}
     override fun Int.foo() {}
 
-    override val String.foo = 0
-    override val Int.foo = 0
+    override val String.foo: Int
+        get() = 0
+    override val Int.foo: Int
+        get() = 0
 
     fun use(s: String) {
         s.foo