[libcxx] [llvm] [libc++] Unify the benchmarks with the test suite (PR #101399)

[Louis Dionne via llvm-commits]

ldionne wrote:

Review note: I will split the first commit into a separate PR once I have addressed all the issues that arise from running the benchmarks as part of the test suite.

https://github.com/llvm/llvm-project/pull/101399