[ms-cxxabi] Move VBTableBuilder from CodeGen over to AST/VTableBuilder.cpp

Summary:
No functionality change.

This code should live here long-term because we should be able to use it
to compute correct vftable names.

It turns out that the most natural way to implement the naming algorithm
is to use a caching layer similar to what we already have for virtual
table info in VTableContext.  Subsequent changes will take advantage of
this to fix PR17748, where we have a vbtable name collision.

Reviewers: majnemer

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2499

llvm-svn: 198380