swift

28a0f835 - Turn `@Sendable` completion handlers into an experimental feature.

Turn `@Sendable` completion handlers into an experimental feature. `@Sendable` on completion handlers imported from Objective-C has been implemented for a while, but has been disabled in production builds due to a number of problems we've encountered with rolling it out. Introduce an experimental feature for `@Sendable` completion handlers so we can iterate on this more before we enable it by default. Part of rdar://85569247, which will cover re-landing this feature.