Allow inner lint attributes on block expressions

[lukas-code]

follow-up to #69675.

This pull request allows lint attributes on blocks like this:

fn main() {
    let _ = {
        #![allow(unused_variables)]
        let x = ();
    };
}

I think this should be fine, because attributes are already allowed on blocks, if they are the tail expression of another block:

// this compiles on stable
fn main() {
    let _ = {{
        #![allow(unused_variables)]
        let x = ();
    }};
}

This pull request does not allow outer attributes on blocks or inner attributes that are not lint attributes:

fn main() {
    let _ = #[allow(unused_variables)] {}; // error

    let _ = {
        #![cfg(TRUE)] // error
    };
}

[rust-highfive]

r? @varkor

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[Centril]

Thank you for your contribution. However, I don't think we should be special casing one type of attribute in terms of stability and this does not seem strongly motivated to me (as there are workarounds). Once we feel comfortable stabilizing stmt_expr_attributes this will also be allowed, but without the special case.

cc @petrochenkov

[petrochenkov]

I agree with the previous comment.