Better error message for private constructors of extensible types

[Gbury]

Currently:

# type exn += private Foobar
let _ = raise Foobar;;
Error: Cannot create values of the private type exn

Which is surprising given that the exn type is not private.

From what I understand, there is a difference between a regular sum type begin private, which means that all its constructors are private and therefore, no value of this type can be constructed (which corresponds to the current error message), and an extensible variant type, where each constructor can be private or not, and where the type being private means that no constructors can be added to the type (but not that all constructors are private, contrary to the regular variant case).

This PR adapts the error message in cases of private constructors of extensible variant types to be:
Cannot use private constructor Foobar to create values of the extensible variant type exn which seems more correct.

[gasche]

I agree this is a good idea in principle and the implementation looks good overall. See minor comments.

[trefis]

This seems like a fine change. But I'd like to see the toplevel tests be replaced by expect tests.

[trefis]

Oh no! @gasche was faster.

Also:

[Gbury]

All review taken into account and Changes added.

[Gbury]

Rebase and squashed.

[gasche]

Good to go if CI passes. Thanks!

(For the record, I'm not fond of "many fine eyes" as a cop-out to writing actual reviewer's names, except in the case where the list would get unreasonably long as it was used in stdlib PRs. If you start contributing more to the compiler, which would be very nice, don't pull that one too often.)

[trefis]

If you start contributing more to the compiler, which would be very nice, don't pull that one too often.

I was the one to suggest it :)

[Gbury]

Travis fails (on i386) on test tests/tool-ocamldep-modalias/'main.ml' though, since it also happens on trunk currently, I suppose it's not this PR's fault.

[trefis]

Yes, that's correct.
I'll merge as is.