[llvm] [libc++] Add a workflow to run libc++ benchmarks on historical commits (PR #208080)

[Louis Dionne via llvm-commits]

ldionne wrote:

I'm going to merge this so I can start testing out the functionality and build other infrastructure on top of it. If y'all have comments on it, please post them and I will be very happy to address them in a follow-up PR. Since this is using `workflow_dispatch`, I don't think there's anything really critical (e.g. related to security) that could affect the patch.

https://github.com/llvm/llvm-project/pull/208080