Add labels for extension properties

#KT-2824 Fixed

compiler/testData/diagnostics/tests/thisAndSuper/thisInPropertyInitializer.kt

@@ -0,0 +1,8 @@
+trait Base {
+    fun foo()
+}
+val String.test: Base = object: Base {
+    override fun foo() {
+        this<!UNRESOLVED_REFERENCE!>@test<!>
+    }
+}