Merge pull request #575 from nielsandriesse/patch-16

Update BidirectionalIndexType for removal of ++ operator

Author: lattner

stdlib/public/core/Index.swift

@@ -288,8 +288,10 @@ extension BidirectionalIndexType {
       return _advanceForward(n)
     }
     var p = self
-    for var i: Distance = n; i != 0; ++i {
-      --p
+    var i: Distance = n
+    while i != 0 {
+      p._predecessorInPlace()
+      i._successorInPlace()
     }
     return p
   }
@@ -300,8 +302,10 @@ extension BidirectionalIndexType {
       return _advanceForward(n, limit)
     }
     var p = self
-    for var i: Distance = n; i != 0 && p != limit; ++i {
-      --p
+    var i: Distance = n
+    while i != 0 && p != limit {
+      p._predecessorInPlace()
+      i._successorInPlace()
     }
     return p
   }