optimize std::vector move assignment

2018-07-26  Marc Glisse  <marc.glisse@inria.fr>

	* include/bits/stl_vector.h (_Vector_impl_data::_M_copy_data): New.
	(_Vector_impl_data::_M_swap_data): Use _M_copy_data.
	(vector::_M_move_assign): Reorder the swaps.

From-SVN: r262998