Editorial: CombineDateAndTimeDuration in DifferenceZonedDateTime is infallible

dateDifference and timeDuration cannot have opposite signs here, so the
operation can't fail. This is ensured by the loop above, which backs up
the intermediate date by one day until the signs match, which is asserted
after the loop exits in step 11.

See: #3023

Author: ptomato

spec/zoneddatetime.html

@@ -1139,7 +1139,7 @@ <h1>
         1. Assert: _success_ is *true*.
         1. Let _dateLargestUnit_ be LargerOfTwoTemporalUnits(_largestUnit_, ~day~).
         1. Let _dateDifference_ be CalendarDateUntil(_calendar_, _startDateTime_.[[ISODate]], _intermediateDateTime_.[[ISODate]], _dateLargestUnit_).
-        1. Return ? CombineDateAndTimeDuration(_dateDifference_, _timeDuration_).
+        1. Return ! CombineDateAndTimeDuration(_dateDifference_, _timeDuration_).
       </emu-alg>
     </emu-clause>