Add performance coverage for builder/incremental/watch scenarios

[DanielRosenwasser]

I'm pretty sure we have no perf suite coverage of the builder, but why not?

Originally posted by @amcasey in #44402 (comment)

There's no good reason except for "it's work", but we really should come up with a way to measure our performance characteristics in the perf suite; however, that's not necessarily an easy thing to accomplish.

Some challenges

1. What are good "representative" codebases?
2. What is a good "representative" set of changes to benchmark for rebuilds?
3. How does this integrate with existing perf tests?

[amcasey]

Haha, "why not?" was "why not kick off a perf run anyway?".

The reason "why not have coverage" is that the space of possible incremental changes is enormous. We'd basically have to stumble upon one that turns out to be interesting (i.e. through a separate investigation) and then add that exact change to the suite.