Implement LWG 2766,

Implement LWG 2766,

Swapping non-swappable types and LWG 2749,
swappable traits for variants.
* include/bits/move.h (swap(_Tp&, _Tp&)): Constrain
with __is_tuple_like.
* include/bits/stl_pair.h (swap(pair<_T1, _T2>&, pair<_T1, _T2>&)):
Add a deleted overload.
* include/bits/unique_ptr.h
(swap(unique_ptr<_Tp, _Dp>&, unique_ptr<_Tp, _Dp>&)): Likewise.
* include/std/array
(swap(array<_Tp, _Nm>&, array<_Tp, _Nm>&)): Likewise.
* include/std/optional
(swap(optional<_Tp>&, optional<_Tp>&)): Likewise.
* include/std/tuple (__is_tuple_like_impl, __is_tuple_like):
Move to type_traits.
(swap(tuple<_Elements...>&, tuple<_Elements...>&)): Add a deleted
overload.
* include/std/type_traits (__is_tuple_like_impl, __is_tuple_like):
New.
(swap(_Tp&, _Tp&)): Constrain with __is_tuple_like.
* include/std/utility (__is_tuple_like_impl): Move to type_traits.
* include/std/variant
(swap(variant<_Types...>&, variant<_Types...>&)):
Add a deleted overload.
* testsuite/20_util/optional/swap/2.cc: Add tests for disabled
swaps.
* testsuite/20_util/pair/swap_cxx17.cc: New.
* testsuite/20_util/tuple/swap_cxx17.cc: Likewise.
* testsuite/20_util/unique_ptr/specialized_algorithms/swap_cxx17.cc:
Likewise.
* testsuite/20_util/variant/compile.cc: Add tests for disabled
swaps.
* testsuite/23_containers/array/specialized_algorithms/swap_cxx17.cc:
New.
* testsuite/23_containers/array/tuple_interface/get_neg.cc: Adjust.
* testsuite/23_containers/array/tuple_interface/tuple_element_neg.cc:
Likewise.

From-SVN: r243120