LWG-3211 std::tuple<> should be trivially constructible

[futuarmo]

LWG-3211 std::tuple<> should be trivially constructible

[miscco]

Has this ABI implications? Otherwise the fix is rather trivial (I blame Casey Carter (@CaseyCarter) for that one)

STL/stl/inc/tuple

Lines 207 to 209 in 9959929

	constexpr tuple() noexcept /* strengthened */ {}
	constexpr tuple(const tuple&) noexcept /* strengthened */ {} // TRANSITION, ABI: should be defaulted

[StephanTLavavej]

As the previous ABI-breaker here, I believe that the default ctor should be OK to change, but we'll double-check of course.