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).

libraries/stdlib/test/reflection/KClassTest.kt

@@ -0,0 +1,45 @@
+/*
+ * 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.
+ */
+
+package test.reflection
+
+import test.*
+import kotlin.reflect.*
+import kotlin.test.*
+
+class KClassTest {
+
+    @Test
+    fun className() {
+        assertEquals("KClassTest", KClassTest::class.simpleName)
+//        assertEquals(null, object {}::class.simpleName) // doesn't work as documented in JDK < 9, see KT-23072
+    }
+
+    @Test
+    fun extendsKClassifier() {
+        assertStaticAndRuntimeTypeIs<KClassifier>(KClassTest::class)
+    }
+
+    @Test
+    fun isInstanceCastSafeCast() {
+        fun <T : Any> checkIsInstance(kclass: KClass<T>, value: Any?, expectedResult: Boolean) {
+            if (expectedResult) {
+                assertTrue(kclass.isInstance(value))
+                assertSame(value, kclass.safeCast(value))
+                assertSame(value, kclass.cast(value))
+            } else {
+                assertFalse(kclass.isInstance(value))
+                assertNull(kclass.safeCast(value))
+                assertFailsWith<ClassCastException> { kclass.cast(value) }
+            }
+        }
+
+        val numberClass = Number::class
+        checkIsInstance(numberClass, 1, true)
+        checkIsInstance(numberClass, 1.0, true)
+        checkIsInstance(numberClass, null, false)
+        checkIsInstance(numberClass, "42", false)
+    }
+}