Swift concurrency back deployment

Doug Gregor, a member of the Swift team at Apple, recently opened a PR that adds an option to build the Swift 5.5 Concurrency libraries for older versions of macOS/iOS. This PR generated a lot of interest amongst the Swift community, with people wondering if Swift's new concurrency model would soon be available for use in older OS versions. In this post to the Swift forums, Doug explains the changes contained in his PR.