Unreserve braced enum variants in value namespace
With this PR braced enum variants (`enum E { V { /*...*/ } }`) no longer take a slot in value namespace, so the special case mentioned in the note in https://github.com/rust-lang/rfcs/blob/master/text/1506-adt-kinds.md#braced-structs is removed.
Report - https://github.com/rust-lang/rust/pull/103578#issuecomment-
1292594900.