Add performance measurements to PR checks?

[allevato]

Based on the comment in #957 (comment).

I'm wondering if there's a way we can set up a GitHub action to do a performance run on PRs. It would be nice to have this automatically reported on every PR somehow, to better avoid regressions.

Can we check out and build main in release mode, then run swift-format over itself a few times with --measure-instructions and compute some stats (mean, std dev), then do the same with the PR, and report the diff? We'd probably want to do this in both format and lint mode.

[ahoppen]

Synced to Apple’s issue tracker as rdar://147492596