libstdc++: Refactor emplace-like functions in std::variant

libstdc++: Refactor emplace-like functions in std::variant

libstdc++-v3/ChangeLog:

	* include/std/variant (__detail::__variant::__emplace): New
	function template.
	(_Copy_assign_base::operator=): Reorder conditions to match
	bulleted list of effects in the standard. Use __emplace instead
	of _M_reset followed by _Construct.
	(_Move_assign_base::operator=): Likewise.
	(__construct_by_index): Remove.
	(variant::emplace): Use __emplace instead of _M_reset followed
	by __construct_by_index.
	(variant::swap): Hoist valueless cases out of visitor. Use
	__emplace to replace _M_reset followed by _Construct.