Instead, use the `%target-swift-5.1-abi-triple` substitution to compile the tests
for deployment to the minimum OS versions required for use of _Concurrency APIs.
values.
Teach ActorIsolation that a `nil` isolated argument is statically nonisolated,
and a reference to GlobalActor.shared statically has global actor isolation.
This change also models arbitrary actor instance isolation using VarDecls
when possible, which allows comparing two ActorIsolation values that may
represent different actor instances. Previously, ActorIsolation was
modeled only by storing the nominal actor type and the parameter index,
so the actor isolation value for two different actors was considered
to be equal. Now, the nominal actor type is only used for isolated `self`
in cases where there is no implicit self parameter decl, such as for
stored properties.