Add @ArrayBuilder to the standard library

[vanvoorden (Rick van Voorden)]

@cal

Since the enum in this case is “just a namespace” and serves no purpose at runtime, what are the technical obstacles to back deploying it, other than that this isn’t implemented yet? Naively it seems trivially back deployable if all the associated static functions are back-deployed and / or emitted into the client.

There might be some clues for you from the support to back-deploy Span et al:

Supporting Span in packages

As we approach the release of Swift 6.2, I would like to provide some information about how to support Span in packages. As you may have noticed in the pre-released compilers, Span and its related types are being added to the standard library with backdeployment support for Darwin-based platforms. While the types are backdeployable, there is a limit to how far back they can be back-deployed, so that there are restrictions both on the compiler and on the deployment. The Span family of types incl…