swift234

Fill out the implementation of AvailabilityAttr.

This includes proper printing support as well as proper platform checking
when seeing if a decl is unavailable. A few other places in the code will
now use AvailabilityAttr::isUnavailable instead of just checking the
is-unavailable-always flag (and not always checking the platform).

No new tests yet because this doesn't include /parsing/ the other fields
of AvailabilityAttr. That will come next, at which point we'll test each
of the cases that has been switched over to use
AvailabilityAttr::isUnavailable.

Part of <rdar://problem/17024498>

Swift SVN r20844