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.