[CIR] Upstream support for virtual base classes

### Overview
Upstream support for virtual base classes in ClangIR.

### Suggested minimal test case
```
class A {
public:
  int a;
  virtual void v() {}
};

class B : public virtual A {
public:
  int b;
  virtual void w();
};
```

### Existing incubator tests
clang/test/CIR/CodeGen/vtt.cpp