[Clang] Improve support for expression messages in `static_assert` (#73234)

- Support non-member functions and callable objects for size and data().
We previously tried to (badly) pick the best overload ourselves, in a
way that would only support member functions. We now leave clang
construct an unresolved member expression and call that, properly
performing overload resolution with callable objects and static
functions, consistent with the logic for `get` calls for structured
bindings.
- Support UDLs as message expression.
- Add tests and mark CWG2798 as resolved