Update _PrefixSequence.next() for removal of ++ operator

Author: nielsandriesse

stdlib/public/core/Sequence.swift

@@ -270,7 +270,7 @@ internal class _PrefixSequence<Base : GeneratorType> : SequenceType, GeneratorTy
 
   internal func next() -> Base.Element? {
     if taken >= maxLength { return nil }
-    ++taken
+    taken += 1
 
     if let next = generator.next() {
       return next