swift234

Don't look for near-miss witnesses for unavailable requirements.

We use unavailable requirements as migration hints, but conformance
checking would try a little too hard to actually satisfy them, even
though they wouldn't actually be used. This led to errors about the
/new/ name not matching the /old/ name.

(We now get /two/ diagnostics for using the old name, but we can fix
that later. Filed rdar://problem/26358593.)

rdar://problem/26313044