swift234

Parse and type-check 'x as? T' conditional cast syntax.

Though we plan to revamp the casting syntax, our general plan is for this form of cast, which does a conditional cast and returns an Optional<T> result, to be the one that survives. Parse the status-quo syntax 'x as? T' and type-check it. While we're here, refresh some fixits for redundant casts that referred to the now defunct 'as T' coercion syntax to completely remove whatever cast was in the source code.

Swift SVN r8805