c++: pack init-capture of unresolved overload [PR102629]

Here we were failing to diagnose that the initializer for the capture pack
is an unresolved overload.  It turns out that the reason we didn't recognize
the deduction failure in do_auto_deduction was that the individual 'auto' in
the expansion of the capture pack was still marked as a parameter pack, so
we were deducing it to an empty pack instead of failing.

	PR c++/102629

gcc/cp/ChangeLog:

	* pt.cc (gen_elem_of_pack_expansion_instantiation): Clear
	TEMPLATE_TYPE_PARAMETER_PACK on auto.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-pack-init7.C: New test.