If you use SwiftStdlibCurrentOS availability, you will be able to
use new types and functions from within the implementation. This
works by, when appropriate, building with the CurrentOS availability
set to the current deployment target.
rdar://150944675
Fix a typo that caused us to not include the correct code for platforms
that use "singlethreaded" concurrency.
Also use `var` not `let` for the computed property in `PlatformExecutorFactory`.
rdar://141348916