[CIR] Upstream support for virtual base classes

[andykaylor]

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

[andykaylor]

#155534
#156757
#156770
#156970