Improve support of PDB as an external layout source

Improve support of PDB as an external layout source

Summary:
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.

Reviewers: rsmith, zturner, rnk, mstorsjo, majnemer

Reviewed By: rnk

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D49871

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@338353 91177308-0d34-0410-b5e6-96231b3b80d8