GetDifferenceSettings: Move call to NegateRoundingMode to a more reasonable step

[anba]

NegateRoundingMode is currently called between two (fallible) ValidateTemporalUnitValue calls. This looks a bit strange.

Suggestion:

• Call NegateRoundingMode directly after GetRoundingModeOption.
• Or call NegateRoundingMode directly before the Return step.

[ptomato]

@sffc @Manishearth We ended up with this order as a result of #3130. Are there any constraints as to whether NegateRoundingMode would need to be on one side of the engine/temporal_rs boundary?

[Manishearth]

I don't really know how to answer that question.

It could be moved to either side, but if it is moved that will be a subtle change that will be really annoying to track and implement. This kind of subtlety is exactly what motivated #3130 in the first place

temporal_rs has it within the temporal_rs boundary, so it occurs after all the validate calls.

[ptomato]

@Manishearth Please correct my understanding if wrong: if we take Anba's second suggestion, that would actually align better with temporal_rs?

[Manishearth]

Yes, I think so.