Fix misleading error messages for keywords that escape a const initializer

[willpuhr]

Fixes #156200

In addition to the ? operator, handles the cases listed below where a keyword would move control flow out of a const initializer. The current error misleadingly states the keyword was used outside its relevant scope.

return inside a function body:

const fn foo() -> Result<(), ()> { Ok(()) }

fn main() {
    const A: () = {
        // error[E0572]: return statement outside of function body
        return;
        ()
    };
}

continue or break inside a loop:

fn main() {
    loop {
        const A: () = {
            // error[E0268]: `continue` outside of a loop
            continue;
            ()
        };
    }
}

break inside a labeled block:

fn main() {
    'a: {
        const A: () = {
            // error[E0767]: use of unreachable label `'a`
            // error[E0268]: `break` outside of a loop or labeled block
            break 'a;
            ()
        };
    }
}

[rust-bors]

☔ The latest upstream changes (presumably #157863) made this pull request unmergeable. Please resolve the merge conflicts.