Actually, introduce static LLCoros::logname() and make the namespaced free
function an alias for that.

Because CoroData is a subclass of LLInstanceTracker with a key, every instance
requires a distinct key. That conflicts with our "getName() returns empty
string for default coroutine on thread" convention. Introduce a new CoroData
constructor, specifically for the default coroutine on each thread, that
initializes the getName() name to empty string while providing a distinct
"mainN" key. Make get_CoroData() use that new constructor for its thread_local
instance, passing an atomic<int> incremented each time we initialize one for a
new thread.

Then LLCoros::logname() returns either the getName() name or the key.