Loc.to_string: bad support for multiple lines (Mantis 4598)

[damiendoligez]

Transferred from http://caml.inria.fr/mantis/view.php?id=4598

When the location extends several lines, the format used by Loc.to_string is not compatible with the way Emacs parses locations, it should be of the form

  File "xxx", line 12-13, characters 34-23:

The following code maybe shows better what I mean

  let (fname, line0, bol0, off0, line1, bol1, off1, _) = Loc.to_tuple loc in
  let line =
    if line0 = line1 then sprintf "%i" line0
    else sprintf "%i-%i" line0 line1 in
  sprintf "File \"%s\", line %s, characters %i-%i"
    fname line (off0 - bol0) (off1 - bol1)