[libc++abi] Look into optimizing dynamic_cast (DAG rightmost) benchmark

The `dynamic_cast (DAG rightmost)` benchmark runs much slower against libc++ than against libstdc++, which means there is room for improvement. While we discourage the use of `dynamic_cast` in performance-sensitive code paths, in practice it still gets used a lot, so having a performant implementation is worth it.

The issue identifies the `dynamic_cast (DAG rightmost)` benchmark but no source file or test. Start by locating and running that benchmark, then compare libc++abi results with libstdc++; done means achieving a measured performance improvement.

Written by the indexing model from the issue text.