Rollup merge of #66183 - Centril:empty-vis-trait-decl, r=petrochenkov

Rollup merge of #66183 - Centril:empty-vis-trait-decl, r=petrochenkov

*Syntactically* permit visibilities on trait items & enum variants

Fixes #65041

Suppose we have `$vis trait_item` or `$vis enum_variant` and `$vis` is a `:vis` macro fragment. Before this PR, this would fail to parse. This is now instead allowed as per language team consensus in https://github.com/rust-lang/rust/issues/65041#issuecomment-538105286. (See added tests for elaboration.)

Moreover, we now also permit visibility modifiers on trait items & enum variants *syntactically* but reject them with semantic checks (in `ast_validation`):

```rust
#[cfg(FALSE)]
trait Foo { pub fn bar(); } // OK

#[cfg(FALSE)]
enum E { pub U } // OK
```