[Bug libstdc++/56785] std::tuple of two elements does not apply empty base class optimization when one of its elements is a std::tuple with two elements

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56785

--- Comment #11 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Created attachment 32994
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=32994&action=edit
stop the inheritance recursion at the last element

This fixes all the issues mentioned here