[Concurrency] Fix availability problem with UnownedTaskExecutor.
a550080 added a call to the new initializer, but that call
came from a function annotated with `StdlibDeploymentTarget 6.2`,
rather than `SwiftStdlib 6.2`, while the initializer was set to
`SwiftStdlib 6.2`. This will fail in some build configurations
(specifically where the target being built for is older than that
implied by `SwiftStdlib 6.2`).
rdar://157217460
