[llvm] Reland: "[Exegesis] Add the ability to dry-run the measurement phase (#121991)" (PR #122775)

[Nico Weber via llvm-commits]

nico wrote:

Also on green dragon, which I think are LLVM's official mac bots: https://green.lab.llvm.org/job/llvm.org/view/All/job/clang-stage1-cmake-RA-incremental/lastCompletedBuild/testReport/

https://github.com/llvm/llvm-project/pull/122775