This attribute creates an unavailable extension with a `Sendable` conformance so that the type is explicity marked as not being `Sendable`.
We also fully suppress diagnostics about unavailable Sendable conformances in Swift 5 mode code. (This is not fully developed yet—it should return to being a warning in concurrent contexts.)
The behavior when a @_nonSendable and a Sendable conformance are both on the same type is also not right yet.
