[llvm] [Exegesis] Add `--dry-run-measurement` (PR #121991)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Could we just add this as a new phase to `--benchmark-phase`? Creating a new flag seems a little weird to me.

This also needs to have support in the subprocess execution mode, or (more likely) at least just a message saying it is not supported.

https://github.com/llvm/llvm-project/pull/121991