c++: partial ordering and dependent operator expr [PR105425]

c++: partial ordering and dependent operator expr [PR105425]

Here ever since r12-6022-gbb2a7f80a98de3 we stopped deeming the partial
specialization #2 to be more specialized than #1 ultimately because
dependent operator expressions now have a DEPENDENT_OPERATOR_TYPE type
instead of an empty type, and this made unify stop deducing T(2) == 1
for K during partial ordering for #1 and #2.

This minimal patch fixes this by making the relevant logic in unify
treat DEPENDENT_OPERATOR_TYPE like an empty type.

	PR c++/105425

gcc/cp/ChangeLog:

	* pt.cc (unify) <case TEMPLATE_PARM_INDEX>: Treat
	DEPENDENT_OPERATOR_TYPE like an empty type.

gcc/testsuite/ChangeLog:

	* g++.dg/template/partial-specialization13.C: New test.

(cherry picked from commit 509fd16da8528444dccc98cef57a18a295c3f1b4)