[libcxx] [llvm] [libc++] Add tools for gathering historical benchmark data (PR #212775)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Overall this looks pretty reasonable.

Some of this looks reasonably unit testable (e.g., `select-anchor-commits` would probably be testable without mocking inside a LLVM checkout), which would help a lot with maintainability.

https://github.com/llvm/llvm-project/pull/212775