Allow an actor to be exposed to Objective-C via `@objc` without
inheriting from `NSObject`, and remove the loophole that allowed
actors to inherit from `NSObject`.
Fixes rdar://78333614
Treat actors as being semantically `final` throughout the type checker.
This allows, for example, a non-`required` initializer to satisfy a
protocol requirement.
We're leaving the ABI open for actor inheritance should we need it.
Addresses rdar://78269551.
The check that limited inference of actor isolation meant that we were
incorrectly computing actor isolation for (e.g.) overrides when
parsing from Swift interfaces. Only limit inference for cases where we
are guaranteed to synthesize an attribute by inference.
