swift234

Extend @availability(*,unavailable) checking to protocol methods.

Protocols can declare methods as being unavailable, as they do
in NSObjectProtocol (e.g., 'retain').  We both need to flag these
uses, but understand this for protocol conformance.  For protocol
conformance, treat unavailable methods as if they were marked
optional.  The compiler will not allow you to use these methods
anyway.

This finishes up support for:

    <rdar://problem/16331335> Ban ObjC ARC entry points

Swift SVN r15644