This was part of the original weak design that
there was never any particular reason to rush the
implementation for. It's convenient to do this now
so that we can use it to implement Unmanaged<T> for
importing CF types.
Swift SVN r16693
The idea for now is that this is a SIL-only type used for
representing the storage of a weak or unowned reference.
Having it be its own type is pretty vital for reasonable
behavior in SIL and IR-generation, and it's likely that
this will surface into runtime metadata as well (hence
the mangling).
I've implemented a bunch of things that technically I don't
think are necessary if this stays out of the typechecker,
but it's easier to implement half-a-dozen "recurse into
the child type" methods now that it would be to find them
all later if we change our minds.
Swift SVN r6091
