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

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

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

Done.

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

yeah I found it more difficult to support this in the subprocess mode, so I simply return an error message if the user try to dry-run in that mode.

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