[clang] [clang][bytecode] Support virtual bases in C++26 (PR #204289)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

What's the plan for non-bytecode?  Do we need some kind of bailout to avoid untested codepaths?

https://github.com/llvm/llvm-project/pull/204289