c++: constantness of non-dependent NTTP argument [PR107437]

c++: constantness of non-dependent NTTP argument [PR107437]

Here we're rejecting the use of the lambda capture of 't' (of empty
type) as a template argument ultimately because convert_nontype_argument
checks constantness using is_constant_expression, which returns false
for lambda captures since want_rval=false.  But in this case I believe
an lvalue-to-rvalue conversion of the argument is implied, so we should
be using is_rvalue_constant_expression instead (which would return true
here).

However, it doesn't seem necessary to consider constantness at all
when deciding whether to instantiate a non-dependent argument in
convert_nontype_argument.  So this patch gets rid of the problematic
constantness test altogether, which incidentally also fixes the similar
dg-ice'd testcase from PR87765.  This is in line with a similar
change we made to finish_decltype_type in r12-7564-gec0f53a3a542e7.

	PR c++/107437
	PR c++/87765

gcc/cp/ChangeLog:

	* pt.cc (convert_nontype_argument): Relax is_nondep_const_expr
	test to !inst_dep_expr_p.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/lambda-generic-107437.C: New test.
	* g++.dg/cpp1z/constexpr-lambda26.C: Remove dg-ice.