c++: unifying lvalue vs rvalue (non-forwarding) ref [PR116710]

When unifying two (non-forwarding) reference types, unify immediately
recurses into the referenced type without first comparing rvalueness.
(Note that at this point forwarding references and other reference
parameters have already been stripped to their referenced type by
maybe_adjust_types_for_deduction, so this code path applies only to
nested reference types.)

	PR c++/116710

gcc/cp/ChangeLog:

	* pt.cc (unify) <case REFERENCE_TYPE>: Compare rvalueness.

gcc/testsuite/ChangeLog:

	* g++.dg/template/unify12.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>