OCaml 5: Backport flambda-backend#1960 (#268)

* flambda-backend: Error message: add hint for unboxed types (#1960)

* generalize hint to all unboxed types

* add regression test

* revert previous change

* extra logic in Ptyp_class

* better error wording

* Fix backport of ocaml-flambda#1960

* Remove mention of now-deprecated t# syntax for poly variant type

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Co-authored-by: alanechang <143514551+alanechang@users.noreply.github.com>