Make the index increment unchecked in UnsafeBufferPointer.Iterator.next

Author: meg-gupta

stdlib/public/core/UnsafeBufferPointer.swift.gyb

@@ -163,7 +163,7 @@ extension UnsafeBufferPointer.Iterator: IteratorProtocol {
     if start == _end._unsafelyUnwrappedUnchecked { return nil }
 
     let result = start.pointee
-    _position  = start + 1
+    _position  = start &+ 1
     return result
   }
 }