Pull "Domain.join preserves the backtrace" from upstream

[dijkstracula]

This patch ports https://github.com/ocaml/ocaml/pull/14363/files , which is #2 but implemented properly, into our fork. (Since we don't run the test suite I elided those files.)

Test plan: This patch was written by OCaml devs so testing here is just meant to ensure I applied the relevant parts of the patch correctly. There were no issues backporting it from the 5.5 trunk so I was not expecting any strange results, however: confirmed before/after behaviour on a trivial program that raises inside a child:

let _ =
  let f () = failwith "in the child domain" in
  match Domain.(spawn f |> join) with
  | v -> ()
  | exception e ->
      let bt = Printexc.get_raw_backtrace () in
      Printexc.raise_with_backtrace e bt

$ # BEFORE: backtrace terminates at the domain join point
$ rm a.out; ocamlopt test.ml && OCAMLRUNPARAM='b=1' ./a.out
Fatal error: exception Failure("in the child domain")
Raised at Stdlib__Domain.join in file "domain.ml", line 299, characters 16-24
Called from Test in file "test.ml", line 3, characters 16-31

$ #AFTER: we observe the full backtrace cross-domain
$ opam pin add ocaml-variants.5.3.0+options "git+file:///Users/ntaylor/code/ocaml#nathan/domain_join_preserves_backtrace_upstream" --update-invariant -y
$  rm a.out; ocamlopt test.ml && OCAMLRUNPARAM='b=1' ./a.out
Fatal error: exception Failure("in the child domain")
Raised at Stdlib.failwith in file "stdlib.ml", line 29, characters 17-33
Called from Stdlib__Domain.spawn.body in file "domain.ml", line 266, characters 16-20
Re-raised at Stdlib__Domain.spawn.body in file "domain.ml", line 282, characters 8-17
Re-raised at Stdlib__Domain.join in file "domain.ml", line 299, characters 22-57
Called from Test in file "test.ml", line 3, characters 16-31

[yosefAlsuhaibani]

Do we care that the msvc build is failing?

[dijkstracula]

The build itself succeeds but for some reason the Windows CI job is not configured correctly so it bombs out when it tries to run unit tests :/