Refactor and simplify class scope name lookup.

Refactor and simplify class scope name lookup.

This is partly in preparation for an upcoming change that can change the
order in which DeclContext lookup results are presented.

In passing, fix some obvious errors where name lookup's notion of a
"static member function" missed static member function templates, and
where its notion of "same set of declarations" was confused by the same
declarations appearing in a different order.