Issue a deprecation warning in Swift 5 and an error in Swift 6 when we encounter @UIApplicationMain and @NSApplicationMain. These attributes are unnecessary now that @main works with UIApplicationDelegate and NSApplicationDelegate. As these conformances are required when working with the corresponding attributes, we can migrate users off of them by replacing them with @main.
This is required to correctly use the mock SDK when the SDK overlay is
built and tested separately. (Otherwise, the mock SDK might not get
used, because the overlay SDK options would expand from the
%-substitution, appear first on the command line, and shadow the mock
SDK in the search path).
Swift SVN r25185
