This came out of today's language review meeting.
The intent is to match #available with the attribute
that describes availability.
This is a divergence from Objective-C.
Swift SVN r28484
Enable checking for uses of potentially unavailable APIs. There is
a frontend option to disable it: -disable-availability-checking.
This commit updates the SDK overlays with @availability() annotations for the
declarations where the overlay refers to potentially unavailable APIs. It also changes
several tests that refer to potentially unavailable APIs to use either #available()
or @availability annotations.
Swift SVN r27272
