swift234

Implement new syntactic sugar for UncheckedOptional<T>.

This leaves in the existing syntax for @unchecked T?.  That will
be addressed in later patches.

There's still a mysterious case where some of the SIL output
includes UncheckedOptional<T> and some places T!.

Moreover, this doesn't handle SourceKit's behavior for printing
for overrides.  This just handles parsing the 'T!' syntax.

Swift SVN r16945