swift234

[NSError bridging] Extract embedded NSError from an Error consistently.

When emitting an existential erasure to Error from an archetype, use
the _getEmbeddedNSError() witness. If it produces an NSError, erase
that; otherwise, go through the normal erasure path.

Of course, make NSError and CFError implement _getEmbeddedNSError() so
this kicks in for the obvious cases as well as the more obscure ones.

Fixes the rest of SR-1562 / rdar://problem/26370984.