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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> @ojhunt Are you OK with this being merged in its current form. I'd like to put off the refactoring I suggested until after the last piece of the dynamic cast handling lands (I have one more patch waiting) so I have a stable, working base to compare against.

yup - as above, there's no reason to block this on a potentially significant refactoring as we've already surrendered to the existence of multiple copies.

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