lint unused labels

[llogiq]

We should lint labels that aren't used anywhere, as in

fn unused_label() {
    'label: for i in 1..2 {
        if i > 4 { continue }
    }
}

To do this, we can use a Visitor to walk any FnDecl, collecting all used labels in a HashSet, then for all loops lint labels whose name isn't in the set. We also probably want to store the function's Block somewhere so we can clear the set in its check_block_post method – or perhaps I'll add another rustc PR with a check_fn_post method for both LintPasses.

[ticki]

I believe this is something rustc should do, since this is a relatively simple task, which will not break any consistencies in the current upstream lints.

[llogiq]

Just a heads-up: We have check_fn_post(..).