Sorry, I wasn't thinking about the Xcode part! The compiler feature is present in Swift 5.0, but Xcode appears to still need to be updated to write its command line arguments to a @params file in order for it to work.
To make sure that the issue is addressed properly, you may want to update your radar to emphasize that it's not a Swift compiler issue anymore, but that Xcode specifically needs to add support for the new behavior.