Restore signature of ChangeOwnerTraverser.change

The result type changed from Unit to Any in PR 10389.

This broke the scalatest assert macro, which uses API from
scala.reflect.internal. It shouldn't, binary compatibility
is not checked in the internal package. Hopefully this commit
fixes it anyway.

src/reflect/scala/reflect/internal/Trees.scala

@@ -1624,7 +1624,7 @@ trait Trees extends api.Trees {
     protected val changedSymbols = mutable.Set.empty[Symbol]
     protected val treeTypes = mutable.Set.empty[Type]
 
-    def change(sym: Symbol) = {
+    def change(sym: Symbol): Unit = {
       if (sym != NoSymbol && sym.owner == oldowner) {
         sym.owner = newowner
         changedSymbols += sym