Lifetime token needs to be considered can_begin_expr

I would expect the expression `'a: while false {}` to be accepted by a $:expr matcher.

```rust
macro_rules! m {
    ($e:expr) => {};
}

fn main() {
    // okay
    let _ = 'a: while false {};

    // no rules expected the token `'a`
    m!('a: while false {});
}
```

cc @kennytm 
