When building a deduction guide from the Test constructor, we need to
rewrite the use of _dummy into a dependent reference, i.e. Test<T>::template
_dummy. We were using SCOPE_REF for both type and non-type templates; we
need to use UNBOUND_CLASS_TEMPLATE for type templates.
PR c++/102123
gcc/cp/ChangeLog:
* pt.cc (tsubst_copy): Use make_unbound_class_template for rewriting
a type template reference.