[PATCH] [ms-cxxabi] Implement vbtable name mangling

[Reid Kleckner]

  I think the code is a little hard to digest without an explanation of the underlying algorithm.  I added a large doc comment to help explain.

  The big question is: can this be done with lookupInBases() and a list of CXXBasePaths?  I don't think so, because MSVC seems to do extend paths while it walks the inheritance graph.  I think Test17 shows this pretty clearly with it's alternating pattern.  I would have to re-discover the underlying graph structure from the list of paths if I used lookupInBases().

http://llvm-reviews.chandlerc.com/D636