`@_nonSendable` types get a synthesized, unavailable extension that
declares the Sendable conformance. This extension also needs to have
appropriate platform availability for the type that is being marked
non-Sendable, so the platform-specific attributes are copied from the
nominal type declaration. However, for nested types, we might need to
copy those attributes from an enclosing declarations. Do so when
appropriate.
Fixes rdar://90330588.
`@_nonSendable` on a type effectively desugars to an unavailable
extension that provides (`@unchecked`) conformance to the `Sendable`
protocol. Make sure we copy over platform availability so that the
extension does not promise greater availability than the type it extends.
Fixes rdar://90330588.