Reland "[mlgo] Hook up the interactive runner to the mlgo-ed passes"

This reverts commit a772f0bb.

The main problem was related to how we handled `dbgs()` from the hosted
compiler. Using explicit `subprocess.communicate`, and not relying on
dbgs() being flushed until the end appears to address the problem.

Also some fixes due to some bots running older pythons, so we can't have
nice things like `int | float` and such.