Progress towards #17957
Because of CoreM, I did not move the DynFlags and HscEnv to other
modules as thoroughly as I usually do. This does mean that risk of
DynFlags "creeping back in" is higher than it usually is.
After we do the same process to the other Core passes, and then figure
out what we want to do about CoreM, we can finish the job started
here.
That is a good deal more work, however, so it certainly makes sense to land this now.