[Concurrency] nonisolated can only be applied to actor properties with Sendable type.

[hborla]

Previously, the compiler allowed nonisolated to be written on any immutable stored property regardless of whether the type conformed to Sendable. Similarly, let properties in isolated types were considered nonisolated within the module regardless of the property type. This is unfortunate for a few reasons:

• nonisolated on a non-Sendable property isn't helpful because you still cannot access the value across isolation boundaries. There were special case Sendable checks for actor lets sprinkled around to handle the diagnostics in these cases.
• There's a semantic difference between accessing a nonisolated let vs a nonisolated(unsafe) var property across isolation domains. From the caller's perspective, nonisolated should have the same semantics regardless of whether or not unsafe was used.

Instead, we should treat lets of non-Sendable type as still being isolated, and ban writing nonisolated on a non-Sendable let property. Programmers can now use nonisolated(unsafe) if they need to access non-Sendable state across isolation boundaries. This also fixes an issue where flow isolation would prevent access to a nonisolated(unsafe) actor property after self becomes nonisolated in an initializer.

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test Linux

[hborla]

@swift-ci please smoke test