[clang] [CIR] Upstream missing support for vtables and virtual bases (PR #192617)

[via cfe-commits]

xiongzile wrote:

Hi @andykaylor,
I've reorganized my changes into two commits:
1. Relative vtable layout support for virtual base adjustment (with tests)
2. A minimal portion of vtable type metadata support
(More detailed description is in the commits)
Could you take a look when you have time? 
Thanks!

https://github.com/llvm/llvm-project/pull/192617