DebugInfo: Do not include implicit members (implicit special members, template instantiations, etc) in the list of members of a structure.

These members will still be lazily added to the relevant DWARF DIEs in
LLVM but when enumerating the members they will not appear. This allows
DWARF type units to be more consistent - the type unit will never
contain these special members (so all instances of the type should have
the same DIEs without some having some special members and others having
others) and the special members will be added to the skeletal
declaration that appears in the relevant compile_unit.