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.
I've preserved this Windows-only test (but also have included other non-execution tests, which may become cross-platform some later, as Pavel has mentioned at D49410), but if you'll find it unnecessary I'll remove it.