Add option to print number of instructions executed by swift-format

[ahoppen]

I’m planning to use this option to measure the performance of swift-format periodically on a fixed set of project to detect any performance regressions.

I made the --measure-instructions option hidden so that it only shows up when running swift-format lint --help-hidden.

[allevato]

Nice! Is there a way we can have these numbers reported in CI, say, once we set up swift-format to verify its own formatting?

I guess what I'd want is to have swift-ci comment back on the PR any performance changes, similar to the toolchain's benchmark runs, but I don't know how difficult that would be to set up (especially since we still don't have triggers on this repo itself).

[ahoppen]

Yes, that would be nice but for that we would also need to have a stable project to format as a baseline. My plan for now is to run these tests locally. I’ll let you know if I discover any performance regressions. Once I have finished the performance testing script, I can also share it with you so you can run it locally before merging PRs that might have performance impact.