Implemented deserialization of references. References are handled

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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@43570 91177308-0d34-0410-b5e6-96231b3b80d8