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/js/Sealed.kt.after

@@ -1,6 +1,6 @@
 // Sealed: to be implemented
 impl sealed class Sealed {
-    object Obj : Sealed()
+    object Obj : Sealed
 
-    class Klass(val x: Int) : Sealed()
+    class Klass(x: Int) : Sealed
 }