[clang] [CIR] Add support for exact dynamic casts (PR #164007)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

The class traversal logic looks to be pretty much identical to the non-CIR final class dynamic_cast optimization (`CGCXXABI::getExactDynamicCastInfo`) is it possible to reuse that logic? maybe extending it if additional information is needed? (also while trying to find that code again I found yet another near copy of the same logic. _sigh_ :D0

https://github.com/llvm/llvm-project/pull/164007