Fix observable call of CalendarDateFromFields in DifferenceTemporalPlainYearMonth

The reference code was out of line with the spec text here, which says
that CalendarDateFromFields must be called the first time before the
second call to DifferenceTemporalPlainYearMonth. These are observable.

See also tc39/proposal-temporal#2252

UPSTREAM_COMMIT=7afcd5d2a49cd38cafac710f394b3c60232fa0a6

Author: ptomato

lib/ecmascript.ts

@@ -4627,12 +4627,12 @@ export function DifferenceTemporalPlainYearMonth(
   const fieldNames = CalendarFields(calendar, ['monthCode', 'year'] as const);
   const otherFields = PrepareTemporalFields(other, fieldNames, []);
   otherFields.day = 1;
-  const thisFields = PrepareTemporalFields(yearMonth, fieldNames, []);
-  thisFields.day = 1;
   // The calls to PrepareTemporalFields don't mark day as a required property,
   // and TS doesn't automatically narrow the type of the object because of the
   // assignments above, so we must "cast" the inputs.
   const otherDate = CalendarDateFromFields(calendar, otherFields as typeof otherFields & { day: number });
+  const thisFields = PrepareTemporalFields(yearMonth, fieldNames, []);
+  thisFields.day = 1;
   const thisDate = CalendarDateFromFields(calendar, thisFields as typeof thisFields & { day: number });
 
   const untilOptions = MergeLargestUnitOption(options, largestUnit);