[Bug libstdc++/78302] is_move_constructible_v<tuple<nonmovable>> should be false

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78302

--- Comment #2 from TC <rs2740 at gmail dot com> ---
Ugh, quasi-dup, I suppose. But if I'm reading the correspondence correctly, the
ABI breaking is only because of triviality, correct?

So if we just want to make is_move_constructible not lie, it should be possible
to do something like adding an empty base at the top level that is either
movable or not depending on __and_<is_move_constructible<_Elements>...> so that
the tuple(tuple&&) =default; will become deleted for nonmovable elements?