Change all uses of x = x.successor() to use x._successorInPlace()

...because it is apparently more efficient in some cases.  Technically
we don't do this in ALL places, because it would be unfortunate if
the implementation of _successorInPlace() were self recursive :-)

stdlib/public/core/StringCharacterView.swift

@@ -143,9 +143,9 @@ extension String.CharacterView : CollectionType {
 
       var gcb0 = graphemeClusterBreakProperty.getPropertyRawValue(
           unicodeScalars[start].value)
-      start = start.successor()
+      start._successorInPlace()
 
-      for ; start != end; start = start.successor() {
+      for ; start != end; start._successorInPlace() {
         // FIXME(performance): consider removing this "fast path".  A branch
         // that is hard to predict could be worse for performance than a few
         // loads from cache to fetch the property 'gcb1'.