MS ABI: Mangle virtual member pointer thunks with the correct CC

Virtual member pointers are implemented using a thunk.  We assumed that
the calling convention for this thunk was always __thiscall for 32-bit
targets and __cdecl for 64-bit targets.  However, this is not the case.
Mangle in whichever calling convention is appropriate for this member
function thunk.

llvm-svn: 232254