Streamline buildkite configuration a bit (#41494)
* Add `/cache/repos` as a mapping into the CI sandbox
This should allow `git` to find its cached objects properly, which
should silence the warnings on CI, and also give us the proper git
version info within buildkite builds
* Break up `llvmpasses` output a bit
* Provide `/cache/repos` for `whitespace` as well
* Give a positive message if whitespace check passes
It's a little unnerving to have a silent command block in buildkite, so
let's output a success message if everything is on the up-and-up
