Implement LWG 2729 for tuple.

Implement LWG 2729 for tuple.

	* include/std/tuple (_Tuple_impl(_Tuple_impl&&)):
	Suppress conditionally.
	(_Tuple_impl(_Tuple_impl<_Idx, _UHead, _UTails...>&&)): Likewise.
	(__is_tuple_impl_trait_impl, __is_tuple_impl_trait): New.
	(_Tuple_impl(const _Head&)): Constrain.
	(_Tuple_impl(_UHead&&)): Likewise.
	(_Tuple_impl(_Tuple_impl&&)): Suppress conditionally.
	(_Tuple_impl(const _Tuple_impl<_Idx, _UHead>&)): Constrain.
	(_Tuple_impl(_Tuple_impl<_Idx, _UHead>&&)): Likewise.
	(operator=(const tuple&)): Enable conditionally.
	(operator=(tuple&&)): Suppress conditionally.
	(operator=(const tuple<_UElements...>&)): Constrain.
	(operator=(tuple<_UElements...>&&)): Likewise.
	(operator=(const tuple&)): Enable conditionally (2-param tuple).
	(operator=(tuple&&)): Suppress conditionally (2-param tuple).
	(operator=(const tuple<_U1, _U2>&)): Constrain.
	(operator=(tuple<_U1, _U2>&&)): Likewise.
	(operator=(const pair<_U1, _U2>&)): Likewise.
	(operator=(pair<_U1, _U2>&&)): Likewise.
	* testsuite/20_util/tuple/element_access/get_neg.cc: Adjust.
	* testsuite/20_util/tuple/tuple_traits.cc: New.

From-SVN: r240323