c++: Fix class NTTP constness handling [PR98810]

c++: Fix class NTTP constness handling [PR98810]

Here, when substituting still-dependent args into an alias template, we see
a non-const type because the default argument is non-const, and is not a
template parm object because it's still dependent.

gcc/cp/ChangeLog:

	PR c++/98810
	* pt.c (tsubst_copy) [VIEW_CONVERT_EXPR]: Add const
	to a class non-type template argument that needs it.

gcc/testsuite/ChangeLog:

	PR c++/98810
	* g++.dg/cpp2a/nontype-class-defarg1.C: New test.