Fix #3250: Make HKLambda a cached type
Fix #3250: Make HKLambda a cached type

This is a partial revert from #3239. #3239 made both HKLambdas and
PolyTypes uncached. It turns out that this breaks things if we do it
for HKLambdas. I have not yet figured out why, however. The test case
in #3250 is a bit too large to be able to tell.

dotty-staging/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,som-snytt/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,lampepfl/dotty,som-snytt/dotty,som-snytt/dotty,som-snytt/dotty,lampepfl/dotty