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

[Oliver Hunt via cfe-commits]

ojhunt wrote:

Removed the ugly conditional branch by making the separate path evaluation accessible from CGExpr and allow it to short circuit the casting logic once we know it will always fail. Given we know this path always fails an alternative might be to just remove it from the optimized path and instead emit a warning.

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