[CIR][CIRGen] Support CodeGen for vbase constructors

[Laity000]

1. Add new cir.vtt.address_point op for visiting the element of VTT to initialize the virtual pointer.
2. Implement getVirtualBaseClassOffset method which provides a virtual offset to adjust to the actual virtual pointer in virtual base.
3. Follow the original clang CodeGen scheme for the implementation of most other parts.