Recommit r183298+r183300 'Get rid of VTableContext::ComputeMethodVTableIndices() and VTableContext::getNumVirtualFunctionPointers()'

In r183298, I've used llvm::SmallPtrSet<..., 8> instead of llvm::SmallVector<..., 8> for NewVirtualFunctionsTy by mistake.
This only manifested when a class had more than 8 virtual functions, which wasn't covered by unit-tests