Support KClass.companionObject and companionObjectInstance

#KT-7636 Fixed

compiler/testData/codegen/boxWithStdlib/reflection/classes/companionObject.kt

@@ -0,0 +1,20 @@
+import kotlin.reflect.*
+import kotlin.test.*
+
+class A {
+    companion object C
+}
+
+fun box(): String {
+    val obj = A::class.companionObject
+    assertNotNull(obj)
+    assertEquals("C", obj!!.simpleName)
+
+    assertEquals(A.C, A::class.companionObjectInstance)
+    assertEquals(A.C, obj.objectInstance)
+
+    assertNull(A.C::class.companionObject)
+    assertNull(A.C::class.companionObjectInstance)
+
+    return "OK"
+}

compiler/tests/org/jetbrains/kotlin/codegen/generated/BlackBoxWithStdlibCodegenTestGenerated.java

@@ -3012,6 +3012,12 @@ public class BlackBoxWithStdlibCodegenTestGenerated extends AbstractBlackBoxCode
                 doTestWithStdlib(fileName);
             }
 
+            @TestMetadata("companionObject.kt")
+            public void testCompanionObject() throws Exception {
+                String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/classes/companionObject.kt");
+                doTestWithStdlib(fileName);
+            }
+
             @TestMetadata("defaultType.kt")
             public void testDefaultType() throws Exception {
                 String fileName = JetTestUtils.navigationMetadata("compiler/testData/codegen/boxWithStdlib/reflection/classes/defaultType.kt");

core/reflection.jvm/src/kotlin/reflect/KClassExtensions.kt

@@ -31,6 +31,24 @@ public val <T : Any> KClass<T>.primaryConstructor: KFunction<T>?
         ((it as KFunctionImpl).descriptor as ConstructorDescriptor).isPrimary
     }
 
+
+/**
+ * Returns a [KClass] instance representing the companion object of a given class,
+ * or `null` if the class doesn't have a companion object.
+ */
+public val KClass<*>.companionObject: KClass<*>?
+    get() = nestedClasses.firstOrNull {
+        (it as KClassImpl<*>).descriptor.isCompanionObject
+    }
+
+/**
+ * Returns an instance of the companion object of a given class,
+ * or `null` if the class doesn't have a companion object.
+ */
+public val KClass<*>.companionObjectInstance: Any?
+    get() = companionObject?.objectInstance
+
+
 /**
  * Returns a type corresponding to the given class with type parameters of that class substituted as the corresponding arguments.
  * For example, for class `MyMap<K, V>` [defaultType] would return the type `MyMap<K, V>`.