Like NSObject, CFType has primitive operations CFEqual and CFHash,
so Swift should allow those types to show up in Hashable positions
(like dictionaries). The most general way to do this was to
introduce a new protocol, _CFObject, and then have the importer
automatically make all CF types conform to it.
This did require one additional change: the == implementation that
calls through to CFEqual is in a new CoreFoundation overlay, but the
conformance is in the underlying Clang module. Therefore, operator
lookup for conformances has been changed to look in the overlay for
an imported declaration (if there is one).
https://bugs.swift.org/browse/SR-2388
Previously AnyHashable would consider SwiftValue to be a subclass of
NSObject (which it is in practice) and return false when trying to
compare an AnyHashable of a SwiftValue box to an AnyHashable of the
unboxed value.
If the Swift error wrapped in a _SwiftNativeNSError box conforms to
Hashable, the box now uses the Swift's conformance to Hashable.
Part of rdar://problem/27574348.
