MS ABI: Fix vftable mangling by using the vbtable name algorithm

Summary:
This merges VFPtrInfo and VBTableInfo into VPtrInfo, since they hold
almost the same information.  With that change, the vbtable mangling
code can easily be applied to vftable data and we magically get the
correct, unambiguous vftable names.

Fixes PR17748.

Reviewers: timurrrr, majnemer

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2893

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202425 91177308-0d34-0410-b5e6-96231b3b80d8