[Bug libstdc++/55713] std::tuple<ElementType> incorrectly is convertible to "ElementType" when it is an empty class

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55713

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Now that we have [[no_unique_address]] I'll make our std::tuple use it (I think
I have a partial implementation in a local branch, that I couldn't compile when
I started work on it).

I don't plan to work on it until stage 3 though, as this is a bug and so fixing
it is valid for stage 3.