Mangle the vbptr offset into pointers to member functions

This can actually be non-zero if you override a function from a virtual
base and you have forced the most_general pointer to member
representation.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205727 91177308-0d34-0410-b5e6-96231b3b80d8