Always generate companion objects for sealed classes

Sealed classes are likely generic sums, which means they
need a companion object to contain the `ordinal` method.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -682,7 +682,7 @@ object desugar {
         }
         companionDefs(companionParent, applyMeths ::: unapplyMeth :: companionMembers)
       }
-      else if (companionMembers.nonEmpty || companionDerived.nonEmpty || isEnum)
+      else if (companionMembers.nonEmpty || companionDerived.nonEmpty || isEnum || mods.is(Sealed))
         companionDefs(anyRef, companionMembers)
       else if (isValueClass) {
         impl.constr.vparamss match {

tests/run/ordinal-innerclass.scala

@@ -33,6 +33,6 @@ object Test extends App {
     }
   }
 
-  trait T1
+  sealed trait T1
   case class A1() extends T1
 }