swift234

Add sugar syntax for @unchecked T?: T!

There is ambiguity in the grammar in the case of 'X as T', where
a '!' following the type 'T' indicates a forced cast.  In this case
we do not parse the '!' as indicating @unchecked optional.

This patch also does not change the pretty-printing of @unchecked T?.
That can come in latter patches as it is clear where the sugared
version should be used.

In the REPL, we see:

(swift) let x : Int! = 42
// x : @unchecked Int?

Part of <rdar://problem/16579799>.

Swift SVN r16493