lwt: ensure enum_set::of is constexpr.
lwt: ensure enum_set::of is constexpr.

This allows using it to initialize const static members.
Message-Id: <c7384b892e76b85fa1ae0411cc7273b3d5e362e8@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla