Fixed KT-1797 No completion for nested class name in extension function definition

#KT-1797 Fixed

idea/testData/completion/basic/common/NestedClassNameForExtension2.kt

@@ -0,0 +1,8 @@
+class Test {
+    public class Nested
+}
+
+fun Test.N<caret>
+}
+
+// EXIST: Nested