DebugInfo: Remove dead code, DICompositeType::addMember(DIDescriptor D)

It's no longer necessary to lazily add members to the DICompositeType
member list. Instead any lazy members (special member functions and
member template instantiations) are added to the parent late based on
their context link, the same way that nested types have always been
handled (never being in the member list - just added to the parent DIE
lazily based on context).

Clang's been updated not to use this function anymore as it improves
type unit consistency by never emitting lazy members in type units.