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/doc/rust.md

@@ -1586,10 +1586,10 @@ pub struct Bar {
     field: int
 }
 
-// Declare a public enum with public and private variants
+// Declare a public enum with two public variants
 pub enum State {
     PubliclyAccessibleState,
-    priv PrivatelyAccessibleState
+    PubliclyAccessibleState2,
 }
 ~~~~