Macro expansions are never considered implicit. They have
valid source locations in their macro expansion buffer, they
do not cause implicit 'self' capture diagnostics, etc. This
fixes an issue where existing code that iterates over an async
sequence in an escaping closure would diagnose a use of implicit
'self' via the '#isolation' argument to 'next()'.
When using `#isolation` within a context that is isolated to a
distributed actor, resolve to an `asLocalActor` access to retrieve the
local actor corresponding to the distributed actor.
