Fix misformatting of let bindings with constraints under 4.06
Fix misformatting of let bindings with constraints under 4.06

Recognize the expanded code pattern introduced by
ocaml/ocaml@fd0dc6a0fbf73323c37a73ea7e8ffc150059d6ff to fix
https://caml.inria.fr/mantis/view.php?id=7344 and format it to code
that the parser will expand to the received parsetree.

ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat,ocaml-ppx/ocamlformat