c++: array bound partial ordering [PR108390]

c++: array bound partial ordering [PR108390]

fold_convert doesn't work with a dependent argument, and problematically
differed from the corresponding fold+build_nop further down in the
function.  So change it to match.

	PR c++/108390

gcc/cp/ChangeLog:

	* pt.cc (unify): Use fold of build_nop instead of fold_convert.

gcc/testsuite/ChangeLog:

	* g++.dg/template/partial-order3.C: New test.