Unwind the stack for caml_stash_backtrace

In hacky ocaml-exception-detection mode, unwind the stack one more level
for caml_stash_backtrace. I did some digging in core_unix and I'm not
actually sure when this changed, but traces are stairstep for the
following trivial async program without this quick patch.

open! Core
open! Async

let rec go () =
    Reader.open_file "a.txt"
    >>= fun a ->
    Writer.open_file "b.txt"
    >>= fun b ->
    let p1 = Reader.lines a in
    Pipe.iter p1 ~f:(fun line ->
        Writer.write_line b line;
        Writer.flushed b)
    >>= fun () ->
    Reader.close a
    >>= fun () ->
    Writer.close b
    >>= fun () ->
    Unix.rename ~src:"b.txt" ~dst:"a.txt"
    >>= fun () ->
    go ()

let command =
    Command.async ~summary:"" (Command.Param.return go)

let () =
  Command_unix.run command

src/trace_writer.ml

@@ -748,6 +748,7 @@ end = struct
           (match symbol with
            | "caml_next_frame_descriptor" -> incr frames_to_unwind
            | "caml_raise_exn" -> unwind_stack t thread_info ~time ~frames_to_unwind (-2)
+           | "caml_stash_backtrace" -> incr frames_to_unwind
            | "caml_raise_exception" ->
              unwind_stack t thread_info ~time ~frames_to_unwind 1
            | _ -> ())