swift234

Use the first element of a struct as a source for
extra inhabitants.

Obviously this should eventually be generalized to
take from any element, but this is good enough to
give us zero-cost abstraction via single-field structs.

Contains some bugfixes for the tuple-extra-inhabitant
changes as well, because test coverage for optional
structs is obviously quite a bit richer than for
optional tuples.

All of this is leading towards unblocking IRGen for
importing CFStringRef as Unmanaged<CFString>!.

Swift SVN r17243