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/doctree.rs

@@ -41,7 +41,7 @@ impl Module {
         Module {
             name       : name,
             id: 0,
-            vis: ast::Private,
+            vis: ast::Inherited,
             where: syntax::codemap::DUMMY_SP,
             attrs      : Vec::new(),
             structs    : Vec::new(),