Match compiler dummy position

We match the OCaml compiler's dummy position for default locations
when using `Location.in_file`.

Signed-off-by: Patrick Ferris <patrick@sirref.org>

src/location.ml

@@ -8,7 +8,8 @@ type t = location = {
 }
 
 let in_file name =
-  let loc = { pos_fname = name; pos_lnum = 1; pos_bol = 0; pos_cnum = -1 } in
+  (* This position matches the compiler's [dummy_pos]. *)
+  let loc = { pos_fname = name; pos_lnum = 0; pos_bol = 0; pos_cnum = -1 } in
   { loc_start = loc; loc_end = loc; loc_ghost = true }
 
 let set_filename loc fn =