…flow isolation.

…rrors

as warnings.

Marking the closure parameter to these inits as `@Sendable` changed the
inferred isolation of closure arguments in actor-isolated contexts, which
caused new effects checker errors when accessing isolated properties and
methods without `await`. Mark these `init`s as `@preconcurrency`, and fix
the effects checker to downgrade those errors to warnings when the context
of the call is `@preconcurrency`.

…Cancel:)`.