FIX #2845: Allow breakpoints on expressions with unlifted type
It turns out we can easily support breakpoints on expressions with unlifted types, by translating case tick# of _ -> e into let f = \s . case tick# of _ -> e in f realWorld# instead of just a plain let-binding. This is the same trick that GHC uses for abstracting join points of unlifted type. In #2845, GHC has eta-expanded the tick expression, changing the result type from IO a to (# State#, a #), which was the reason the tick was suddenly being ignored. By supporting ticks on unlifted expressions we can make it work again, although some confusion might arise because _result will no longer be available (it now has unboxed-tuple type, so we can't bind it in the environment). The underlying problem here is that GHC does transformations like eta-expanding the tick expressions, and there's nothing we can do to prevent that.