c++: local class in nested generic lambda [PR109241]

In this testcase, the tree walk to look for bare parameter packs was
confused by finding a type with no TREE_BINFO.  But it should be fine that
it's unset; we already checked for unexpanded packs at parse time.

I also tried doing the partial instantiation of the local class, which is
probably the long-term direction we want to go, but for stage 4 let's go
with this safer change.

	PR c++/109241

gcc/cp/ChangeLog:

	* pt.cc (find_parameter_packs_r): Handle null TREE_BINFO.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/lambda-generic-local-class2.C: New test.