bitset: set_range: mark constructor constexpr
bitset: set_range: mark constructor constexpr

Clang 12 complains if it isn't, not in Seastar code, but still this
can be marked constexpr at no cost.

Reviewed-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <ea03da263b7745644a9be17e72ad6a66a53dee6e@scylladb.com>

avikivity/seastar,scylladb/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,avikivity/seastar,syuu1228/seastar