[SE-0313] Require nonisolated declarations to involve Sendable types. (#1397)
[SE-0313] Require nonisolated declarations to involve Sendable types. (#1397)

Eliminate a potential source of data races by requiring that a
non-isolated declaration only involve `Sendable` types. Otherwise,
the `Sendable` type could be passed across actor boundaries through
nonisolated declarations.
natecook1000/swift-evolution,abertelrud/swift-evolution,abertelrud/swift-evolution,apple/swift-evolution,apple/swift-evolution,natecook1000/swift-evolution