Make common KClass extend common KClassifier

It already does that in all platforms.
Add KClass common API test.

libraries/stdlib/src/kotlin/reflect/KClass.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -13,7 +13,7 @@ package kotlin.reflect
  *
  * @param T the type of the class.
  */
-public expect interface KClass<T : Any> {
+public expect interface KClass<T : Any> : KClassifier {
     /**
      * The simple name of the class as it was declared in the source code,
      * or `null` if the class has no name (if, for example, it is a class of an anonymous object).