Like the change for `open let`, this is only a warning in Swift 4
mode, because it didn't actually cause any harm in practice. Neither a
Swift developer nor a compiler developer wants to rely on that going
forward, though!
...and add a verifier check to make sure we don't mix 'open' and
'final' in the future.
You could /write/ 'open let' in Swift 4.1, but you couldn't actually
override such a property (within the module or outside). To maintain
source compatibility, this is just going to emit a warning before
Swift 5 (instead of an error), since it wasn't harming anything in
practice.
