Disallow extension properties with backing fields

#KT-1682 Fixed

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

@@ -2,7 +2,7 @@
 package outer
 
 fun Int?.optint() : Unit {}
-val Int?.optval : Unit = Unit.VALUE
+val Int?.optval : Unit get() = Unit.VALUE
 
 fun <T: Any, E> T.foo(<!UNUSED_PARAMETER!>x<!> : E, y : A) : T   {
   y.plus(1)

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

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

@@ -1,6 +1,6 @@
 object O
 
 fun Any.foo() = 42
-val Any?.bar = 239
+val Any?.bar: Int get() = 239
 
 val x = O.foo() + O.bar