c++: Fix SFINAE for invalid non-type tparm types.

Just missing the usual SFINAE pattern here.  This was fixed for GCC 10 as a
drive-by in r277902.

gcc/cp/ChangeLog
2020-03-05  Jason Merrill  <jason@redhat.com>

	PR c++/90338
	* pt.c (invalid_nontype_parm_type_p): Check complain for non-literal
	and mutable errors.