c++: generic lambda forwarding function [PR94546]

While instantiating test(Plot) we partially instantiate the generic lambda.
We look at forward<T>(rest)... and see that it's just replacing parameter
packs with new parameter packs and tries to do a direct substitution.  But
because register_parameter_specializations had built up a
NONTYPE_ARGUMENT_PACK around the new parameter pack, the substitution
failed.  So let's not wrap it that way.

gcc/cp/ChangeLog
2020-04-22  Jason Merrill  <jason@redhat.com>

	PR c++/94546
	* pt.c (register_parameter_specializations): If the instantiation is
	still a parameter pack, don't wrap it in a NONTYPE_ARGUMENT_PACK.
	(tsubst_pack_expansion, tsubst_expr): Adjust.