Inconsistent autocompletion in block macro arguments

This is a similar issue as #8158 but not the same.

```rs
macro_rules! some_macro {
    ($block:block) => {
        $block
    }
}

fn main() {
    some_macro!({
        let foo = String::new();

        // typing foo.l here suggests nothing

        foo.len();

        // typing foo.l here suggests len()

    });
}
```