[PDB] Parse UDT symbols and pointers to members (combined patch)

Summary:
In this patch I've tried to combine the best ideas from D49368 and D49410,
so it implements following:

- Completion of UDTs from a PDB with a filling of a layout info;
- Pointers to members;
- Fixes the bug relating to a virtual base offset reading from `vbtable`.
  The offset was treated as an unsigned, but it can be a negative sometimes.
- Support of MSInheritance attribute

Reviewers: asmith, zturner, rnk, labath, clayborg, lldb-commits

Reviewed By: zturner

Subscribers: aleksandr.urakov, stella.stamenova, JDevlieghere, lldb-commits

Differential Revision: https://reviews.llvm.org/D49980

llvm-svn: 339649