re PR libstdc++/56785 (std::tuple of two elements does not apply empty base class optimization when one of its elements is a std::tuple with two elements)

	PR libstdc++/56785
	* include/std/tuple (_Tuple_impl): Remove zero-element specialization
	and define one-element specialization.
	* testsuite/20_util/tuple/56785.cc: New.

From-SVN: r219785