gccrs: Run block cleanup for unlabeled break and continue

[Lishin1215]

This PR builds on the existing HIR::BlockExpr TRY_FINALLY_EXPR cleanup infrastructure.
It keeps unlabeled break and continue inside the block body, so block cleanup
can run when they leave scopes.

It covers these unlabeled cases:

1. continue

loop {
    let _x = Droppable;
    continue;
}

2. tail-position continue

loop {
    let _x = Droppable;
    continue
}

3. break

loop {
    let _x = Droppable;
    break;
}

4. tail-position break

loop {
    let _x = Droppable;
    break
}

5. break with a value

let x = loop {
    let _x = Droppable;
    break 123;
};

The tests use loop instead of while to keep the coverage focused on cleanup behavior.

---

Follow-up work:

• Labeled break / continue
• Tail-position return cleanup

While testing this change, I also found a separate issue that should be
tracked outside this PR:

1. A loop-body local can be dropped twice in some cases. This likely needs
   drop/initialized flags.

fn test_continue_double_drop() {
    let mut done = false;

    loop {
        if done {
            break;
        }

        let _outer = Droppable;

        {
            let _inner = Droppable;
            done = true;
            continue;
        }
    }
}

[Lishin1215]

This handles tail-position control flow like { break } and { continue }.
The generated jump still needs to be added to the block body, otherwise it is
created but never emitted into the final block.

[Lishin1215]

I also opened an issue for the double-drop case I mentioned in the PR description:
#4717