Implemented deserialization of references. References are handled

just like pointers, except that they cannot be backpatched.  This
means that references are essentially non-owning pointers where the
referred object must be deserialized prior to the reference being
deserialized.  Because of the nature of references, this ordering of
objects is always possible.

Fixed a bug in backpatching code (returning the backpatched pointer
would accidentally include a bit flag).

llvm-svn: 43570