[CIR] Upstream support for virtual functions

[andykaylor]

Overview

Upstream support for declaring, calling, and emitting virtual functions in CIR.

This is probably going to involve a lot of changes, including the introduction of vtables. The changes should be upstreamed in the smallest pieces possible and growing the support incrementally.

Suggested minimal test case

class C {
  int a; // Necessary until we have better handling for zero-member classes
  virtual void vf();
}

int f() {
  C c;
}

Existing incubator tests

clang/test/CIR/CodeGen/vtable-emission.cpp