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

src/librustdoc/html/format.rs

@@ -507,7 +507,6 @@ impl fmt::Show for VisSpace {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self.get() {
             Some(ast::Public) => write!(f.buf, "pub "),
-            Some(ast::Private) => write!(f.buf, "priv "),
             Some(ast::Inherited) | None => Ok(())
         }
     }