swift234

Change ImplicitlyUnwrappedOptional to be an enum type itself,
not a struct wrapping an Optional.

Among other things, this means you can now pattern-match on
an IUO.  It also makes it more convenient to build and destroy
them.

SILGen's type lowering should probably canonicalize one kind
of optional to the other so that we don't get silly abstraction
costs from conversion.

Swift SVN r18991