Proposal: make instatiation of a private record/variant raise a warning instead of an error

[tsnobip]

Today, when you try to create a private record or variant, it errors:

module Foo = {
  type t = private {foo: int}
}

let bar = {Foo.foo: 9} // Error: Cannot create values of the private type Foo.t

Though, there are cases where you'd want to force the instantiation for example for testing, moving this from an error to a warning would allow this. We could use the same warning when casting to a private record/variant (cf #8457)

[fhammerschmidt]

Should it be an error that you can downgrade to a warning instead?
Or a warning by default that you can promote to an error?

[tsnobip]

@fhammerschmidt I think it should be an error by default.