[patch] libstdc++/56785 reduce space overhead of nested tuples

[Jonathan Wakely]

This replaces the current empty _Tuple_impl that terminates the
recursive inheritance hierarchy, instead adding the extra code to the
last base class that holds data so that the recursion terminates there
instead.

The purpose of this is to avoid nested tuples having two instances of
the same _Tuple_impl<N> base class, which cannot be placed at the same
address and so take up space despite being empty.

Tested x86_64-linux, committed to trunk.
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