Change MergeLargestUnitOption to match spec

This was previously done with the spread syntax, but we are going to make
a change to it, so we'll need a function instead. The spec text has the
operation MergeLargestUnitOption, so name it that.

The ObjectCreate(null) step was previously missing, so add it in order to
be compliant with the spec.

UPSTREAM_COMMIT=1660e23ae982d84ff7958c85fb44ae981e315d59

Author: ptomato

lib/ecmascript.ts

@@ -1129,6 +1129,15 @@ export function LargerOfTwoTemporalUnits<T1 extends Temporal.DateTimeUnit, T2 ex
   return unit1;
 }
 
+function MergeLargestUnitOption<T extends Temporal.DateTimeUnit>(
+  optionsParam: Temporal.DifferenceOptions<T> | undefined,
+  largestUnit: T
+): Temporal.DifferenceOptions<T> {
+  let options = optionsParam;
+  if (options === undefined) options = ObjectCreate(null);
+  return { ...options, largestUnit };
+}
+
 export function ToPartialRecord<B extends AnyTemporalLikeType>(bagParam: B, fieldsParam: ReadonlyArray<keyof B>) {
   // External callers are limited to specific types, but this function's
   // implementation uses generic property types. The casts below (and at the
@@ -4098,7 +4107,7 @@ function DifferenceISODateTime(
   const date1 = CreateTemporalDate(y1, mon1, d1, calendar);
   const date2 = CreateTemporalDate(y2, mon2, d2, calendar);
   const dateLargestUnit = LargerOfTwoTemporalUnits('day', largestUnit);
-  const untilOptions = { ...options, largestUnit: dateLargestUnit };
+  const untilOptions = MergeLargestUnitOption(options, dateLargestUnit);
   // TODO untilOptions doesn't want to compile as it seems that smallestUnit is not clamped?
   // Type 'SmallestUnit<DateTimeUnit> | undefined' is not assignable to type
   //      'SmallestUnit<"year" | "month" | "day" | "week"> | undefined'.
@@ -4274,7 +4283,7 @@ export function DifferenceTemporalPlainDate(
   if (operation === 'since') roundingMode = NegateTemporalRoundingMode(roundingMode);
   const roundingIncrement = ToTemporalRoundingIncrement(options, undefined, false);
 
-  const untilOptions = { ...options, largestUnit };
+  const untilOptions = MergeLargestUnitOption(options, largestUnit);
   let { years, months, weeks, days } = CalendarDateUntil(calendar, plainDate, other, untilOptions);
 
   if (smallestUnit !== 'day' || roundingIncrement !== 1) {
@@ -4511,7 +4520,7 @@ export function DifferenceTemporalPlainYearMonth(
   const otherDate = CalendarDateFromFields(calendar, { ...otherFields, day: 1 });
   const thisDate = CalendarDateFromFields(calendar, { ...thisFields, day: 1 });
 
-  const untilOptions = { ...options, largestUnit };
+  const untilOptions = MergeLargestUnitOption(options, largestUnit);
   let { years, months } = CalendarDateUntil(calendar, thisDate, otherDate, untilOptions);
 
   if (smallestUnit !== 'month' || roundingIncrement !== 1) {
@@ -4598,7 +4607,7 @@ export function DifferenceTemporalZonedDateTime(
           'or smaller because day lengths can vary between time zones due to DST or time zone offset changes.'
       );
     }
-    const untilOptions = { ...options, largestUnit };
+    const untilOptions = MergeLargestUnitOption(options, largestUnit);
     ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } =
       DifferenceZonedDateTime(ns1, ns2, timeZone, calendar, largestUnit, untilOptions));
     ({ years, months, weeks, days, hours, minutes, seconds, milliseconds, microseconds, nanoseconds } = RoundDuration(