CWG3123  Global lookup for begin and end for expansion statements

As currently specified, `std::tuple` is considered expansion-iterable (which causes a failure when actually expanding), but it should be considered expansion-destructurable instead.

CWG selected an approach for CWG3123 that solves the issue, but has a set of drawbacks different from the currently-specified approach.

EWG is asked to confirm CWG's decision or offer a different approach.