c++: variadic lambda template and empty pack [PR97246]

c++: variadic lambda template and empty pack [PR97246]

In get<0>, Is is empty, so the first parameter pack of the lambda is empty,
but after the fix for PR94546 we were wrongly associating it with the
partial instantiation of 'v'.

gcc/cp/ChangeLog:

	PR c++/97246
	PR c++/94546
	* pt.c (extract_fnparm_pack): Check DECL_PACK_P here.
	(register_parameter_specializations): Not here.

gcc/testsuite/ChangeLog:

	PR c++/97246
	* g++.dg/cpp2a/lambda-generic-variadic21.C: New test.