Ask a frame builder (hopefully the good Mr. Walker or Thylacine will weigh in here) for a competent answer. In my non-expert opinion, however, I'm inclined to say do a light weld along the crack and cover it up as best you can.
Pro: the top tube is one of the least stressed tubes in a diamond frame bicycle.
Con: Tube joints are the most stressed area on any tube.
Did the frame come like this? Is there a chance that this damage occurred in a wreck? If so, then it's quite likely that simply brazing or welding it will be sufficient to get it to hold together even if it's no longer raceworthy.