rustc: Remove private enum variants

This removes the `priv` keyword from the language and removes private enum
variants as a result. The remaining use cases of private enum variants were all
updated to be a struct with one private field that is a private enum.

RFC: 0006-remove-priv

Closes #13535
syntax: Demote `priv` to a reserved keyword

It is no longer used in rust anywhere.

RFC: 0006-remove-priv