*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
```
