Add an option to build the concurrency library for back deployment

Implemented by Doug Gregor, a member of the Swift team at Apple, this unexpected pull request adds an option to build the new Swift Concurrency libraries for older versions of macOS/iOS. These libraries can then be embedded in targets deployed before macOS 12/iOS 15 to support the new concurrency features. I'm still not sure if this makes the new concurrency features fully available when working with older OS versions, but this is something to watch.

Codable synthesis for enums with associated values

Swift 4 introduced support for automatically synthesizing conformance to the Encodable and Decodable protocols. It's a useful feature, most likely used by any application which handles JSON-based network requests. This Swift evolution pitch from Dario Rexin proposes extending auto-synthesis of these conformances to enums with associated values. I'm sure we will see this in an upcoming Swift version.