2019-11-15 Edward Smith-Rowland <3dw4rd@verizon.net> Implement the <tuple> part of C++20 p1032 Misc constexpr bits. * include/std/tuple (_Head_base, _Tuple_impl(allocator_arg_t,...) (_M_assign, tuple(allocator_arg_t,...), _Inherited, operator=, _M_swap) (swap, pair(piecewise_construct_t,): Constexpr. * (__uses_alloc0::_Sink::operator=, __uses_alloc_t): Constexpr. * testsuite/20_util/tuple/cons/constexpr_allocator_arg_t.cc: New test. * testsuite/20_util/tuple/constexpr_swap.cc : New test. * testsuite/20_util/uses_allocator/69293_neg.cc: Extra error for C++20. * testsuite/20_util/uses_allocator/cons_neg.cc: : Extra error for C++20. From-SVN: r278331
