Support for anonymous structs/unions in FieldList

[qwerty01]

Currently there doesn't seem to be a way to determine whether a MemberType is directly part of the type it corresponds to or is part of an anonymous struct/union. If the PDB format includes some kind of tag specifying anonymous types, it would be useful to have that included. If not, a heuristic that seems to work is to track the offsets, lengths, and positions of each field, and use them as constraints for determining the anonymous struct/union layout.