c++: default template arg, partial ordering [PR105481]

The default argument code in type_unification_real was assuming that all
targs we've deduced by that point are non-dependent, but that's not the case
for partial ordering.

	PR c++/105481

gcc/cp/ChangeLog:

	* pt.cc (type_unification_real): Adjust for partial ordering.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/fntmpdefarg-partial1.C: New test.