Return null from Spliterator.getComparator() for natural ordering (fixes #6187)

[holly-agyei]

Fixes #6187.

Problem

Per the Spliterator.getComparator() contract, implementations whose source is sorted in natural order must return null. The JDK uses this signal to clear the SORTED characteristic when the comparator is non-null (StreamOpFlag line 751-755), which in turn disables Stream.sorted()'s short-circuit optimization, causing unnecessary re-sorting of already-sorted data.

ImmutableSortedSet.spliterator() and CollectSpliterators.indexed() (used by ImmutableSortedAsList.spliterator(), which in turn backs RegularImmutableSortedSet.spliterator() and its subset views) returned Ordering.natural() instead of null for naturally-ordered sources, so code like:

ImmutableSortedSet.of(1, 2, 3, 4).stream().sorted().collect(toList());

incurred a full re-sort rather than the documented no-op.

Fix

• Added CollectSpliterators.isNaturalOrder(Comparator), a shared helper that recognizes the two well-known natural-ordering singletons (Ordering.natural() and Comparator.naturalOrder()) as well as null.
• CollectSpliterators.indexed() and ImmutableSortedSet.spliterator() now normalize natural-ordering comparators to null at the getComparator() boundary. Custom and reverse-order comparators are preserved unchanged, so DescendingImmutableSortedSet and user-supplied comparators are unaffected.

@kevinb9n called out special-casing Comparator.naturalOrder() in addition to Ordering.natural() on #6188 (the earlier, stale attempt by @kilink); this PR honors that and adds the test coverage that was missing from both #6188 and the withdrawn #8174.

Tests

Added regression tests at two levels:

CollectSpliteratorsTest

• testIsNaturalOrder_null, _orderingNatural, _comparatorNaturalOrder — helper returns true for each natural-ordering form
• testIsNaturalOrder_reverseOrder_false, _customComparator_false — non-natural comparators are rejected
• testIndexedSortedSpliterator_naturalOrderReturnsNull, _comparatorNaturalOrderReturnsNull — indexed spliterator surfaces null for natural ordering and keeps SORTED
• testIndexedSortedSpliterator_customComparatorIsPreserved — custom comparator round-trips unchanged

ImmutableSortedSetTest

• testSpliteratorGetComparator_naturalOrdering_returnsNull
• testSpliteratorGetComparator_comparatorNaturalOrder_returnsNull
• testSpliteratorGetComparator_customOrdering_returnsComparator
• testSpliteratorHasSortedCharacteristic
• testAsListSpliteratorGetComparator_naturalOrdering_returnsNull (covers the ImmutableSortedAsList path used by RegularImmutableSortedSet)
• testAsListSpliteratorGetComparator_customOrdering_returnsComparator

Test plan

• ./mvnw -pl guava -am install -DskipTests — builds clean
• ./mvnw -pl guava-tests test -Dtest='CollectSpliteratorsTest,ImmutableSortedSetTest' — 17,293 tests, 0 failures
• Broader sweep: ./mvnw -pl guava-tests test -Dtest='*SortedSet*,*SortedMap*,*SortedMultiset*,*Collect*Test,ImmutableListTest,ImmutableSetTest,StreamsTest' — 455,720 tests, 0 failures

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