swift234

Sema: Allow types to conform to protocols that are less available than the type itself.

The previous rules for availability checking prevented types from conforming to protocols
that were less available than the type itself. This was too restrictive, because we want
to allow developers to add additional behavior to their existing classes to
conform to new protocols while still employing older functionality from those classes on
older OSes:

@available(iOS 9.0, *)
protocol UIRefrigeratorDelegate { }

@available(iOS 8.0, *)
class MyViewController : UIViewController, UIRefrigeratorDelegate { }

We now support this idiom (and adding conformnces via protocol extensions) by allowing
references to potentially unavailable protocols inside inheritance clauses.

It is still an availability error to refer to potentially unavailable protocols in other
settings -- so, for example, attempting to cast to an unavailable protocol will cause
a compile-time error.

rdar://problem/21718497

Swift SVN r29958