Changed the way the expression parser handles variables

of reference types.  Previously, such variables were
materialized as references to those references, which
caused undesried behavior in Clang and was useless anyway
(the benefit of using references to variables is that it
allows expressions to modify variables in place, but for
references that's not required).

Now we just materialize the references directly, which
fixes a variety of expressions that use references.

llvm-svn: 143137