Treat nested class of header class as header

Note that the quick fix to implement header class works incorrectly when
that class has nested classes at the moment; this should be fixed
separately

 #KT-15494 Fixed
 #KT-18573 Fixed

idea/testData/multiModuleQuickFix/sealed/header/header.kt

@@ -1,7 +1,7 @@
 // "Create header class implementation for platform JS" "true"
 
 header sealed class <caret>Sealed {
-    object Obj : Sealed()
+    object Obj : Sealed
 
-    class Klass(val x: Int) : Sealed()
+    class Klass(x: Int) : Sealed
 }