The _Tuple_impl base class can be used to disable copy/move assignment,
without requiring an extra base class.

Exception specifications on std::tuple assignment and swap functions can
be defined directly using is_nothrow_swappable, instead of querying the
base classes.

	PR libstdc++/86963
	* include/std/tuple (_Tuple_impl::operator=): Define as deleted.
	(_Tuple_impl::_M_assign): New functions to perform assignment instead
	of assignment operators.
	(_Tuple_impl::_M_swap): Remove exception specification.
	(_Tuple_impl<_Idx, _Head>): Likewise.
	(_TC::_NonNestedTuple, _TC::_NotSameTuple): Use __remove_cvref_t.
	(__tuple_base): Remove.
	(tuple, tuple<_T1, _T2>): Remove inheritance from __tuple_base.
	(tuple::operator=, tuple<_T1, _T2>::operator=): Call _M_assign.
	(tuple::swap, tuple<_T1, _T2>::swap): Define exception specification
	using __is_nothrow_swappable.
	(tuple<_T1, _T2>::tuple(_U1&&, _U2&&)): Use __remove_cvref_t.

From-SVN: r263661