This patch improves support of PDB as an external layout source in the next cases:
- Multiple non-virtual inheritance from packed base classes. When using external layout, there's no need to align NonVirtualSize of a base class. It may cause an overlapping when the next base classes will be layouted (but there is a slightly different case in the test because I can't find a way to specify a base offset);
- Support of nameless structs and unions. There is no info about nameless child structs and unions in Microsoft cl-emitted PDBs. Instead all its fields are just treated as outer structure's (union's) fields. This also causes a fields overlapping, and makes it possible for unions to have fields located at a non-zero offset.