[clang] [clang] Fix crash in dynamic_cast final class optimization (PR #152076)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

I don't have as good of a grasp on a couple of the things this touches, so hopefully @efriedma-quic or @rjmccall can take a look here.  But I saw nothing to comment on.

https://github.com/llvm/llvm-project/pull/152076