Conform functions to extension functions and vice versa

#KT-5989 Fixed

idea/idea-completion/testData/basic/multifile/NotImportedExtensionFunction2/NotImportedExtensionFunction2.dependency.kt

@@ -6,11 +6,11 @@ fun (() -> Unit)?.helloFun1() {
 fun Function0<Unit>.helloFun2() {
 }
 
-fun ExtensionFunction0<String, Unit>.helloFun3() {
+fun @extension Function1<String, Unit>.helloFun3() {
 }
 
 fun Function1<String, Unit>.helloFun4() {
 }
 
 fun Any.helloAny() {
-}
+}