* include/bits/enable_special_members.h: Make
	_Enable_default_constructor constexpr.
	* include/std/variant (variant::emplace, variant::swap, std::swap,
	std::hash): Sfinae on emplace and std::swap; handle __poison_hash bases
	of duplicated types.
	* testsuite/20_util/variant/compile.cc: Add tests.
	* testsuite/20_util/variant/hash.cc: Add tests.

From-SVN: r243294