Raw syntax for lifetimes and loop labels

We don't currently accept:

```rust
pub fn label() {
    'r#async: loop {
        break 'r#async;
    }
}

pub fn lifetime<'r#async>() {}
```

Should we?  This has relevance to edition migrations.

See:

- https://github.com/rust-lang/rust/issues/125986