[libc++] Add a job that runs the benchmarks as part of the CI

This will make it easier to see the performance impact of individual
patches. This is a first step towards better performance tracking, but
many things still need to be figured out:
- We should get a dedicated set of machines to run these tests on
- We could probably reuse regularly-generated baseline results published
  to LNT instead of re-generating the baselines in this job every time.

Author: ldionne