Bracing for match arm doesn't include comment

Before:

        Referrer::Client =>
            // it should be impossible for referrer to be anything else during fetching
            // https://fetch.spec.whatwg.org/#concept-request-referrer
           unreachable!()

After:

        Referrer::Client =>
        // it should be impossible for referrer to be anything else during fetching
        // https://fetch.spec.whatwg.org/#concept-request-referrer
        {
            unreachable!()
        },

Start by reproducing the formatting of the Rust match arm shown in the issue, then trace rustfmt's match-arm bracing and comment-handling logic. Done means comments stay outside the braces and the formatted output matches the After example, including the trailing comma.