Lowering: Fix multiplicity error

[wangandi]

Closes MaterializeInc/database-issues#2209

exists(bar).applied_to(foo), when bar is "simple", was being converted to:

bar.applied_to(foo).distinct_by(foo_cols).map(true) 
union
(foo join 
   (distinct(foo) - (bar.applied_to(foo)).distinct_by(foo_cols))
).map(false)

This caused the loss of all but one copy of each row for where exists(bar).

The correct conversion should be

(foo join 
  bar.applied_to(foo).distinct_by(foo_cols)
).map(true) 
union
(foo join 
   (distinct(foo) - (bar.applied_to(foo)).distinct_by(foo_cols))
).map(false)

[frankmcsherry]

I don't understand this code, unfortunately! It seems to pass tests, and I could imagine it is correct. @benesch might know more about it!

[benesch]

Er, I think you added this code, @frankmcsherry, in #5651! Seems good though.

[frankmcsherry]

I don't think I did. It was added in #5651 though I think by you. In any case, I don't understand it! :D