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
