[libcxx] [llvm] [libc++] Add a job that runs the benchmarks as part of the CI (PR #157567)

[Louis Dionne via llvm-commits]

ldionne wrote:

/libcxx-bot benchmark libcxx/test/benchmarks/join_view.bench.cpp libcxx/test/benchmarks/hash.bench.cpp

https://github.com/llvm/llvm-project/pull/157567