Convert BitSet.toSet

Fix compiler error with Scala 2.13 collections rewrite:

ambiguous reference to overloaded definition,
both method map in trait StrictOptimizedIterableOps of type [B](f: Int => B)scala.collection.immutable.Set[B]
and  method map in trait SortedSetOps of type [B](f: Int => B)(implicit evidence$4: Ordering[B])scala.collection.immutable.SortedSet[B]
match argument types (Int => scala.collection.immutable.BitSet)
    (Q map f).foldLeft(immutable.BitSet.empty)(_ ++ _)
       ^

Author: ashawley

shared/src/main/scala/scala/xml/dtd/impl/NondetWordAutom.scala

@@ -48,7 +48,7 @@ private[dtd] abstract class NondetWordAutom[T <: AnyRef] {
   def nextDefault(Q: immutable.BitSet): immutable.BitSet = next(Q, default)
 
   private def next(Q: immutable.BitSet, f: (Int) => immutable.BitSet): immutable.BitSet =
-    (Q map f).foldLeft(immutable.BitSet.empty)(_ ++ _)
+    Q.toSet.map(f).foldLeft(immutable.BitSet.empty)(_ ++ _)
 
   private def finalStates = 0 until nstates filter isFinal
   override def toString = {