Unable to modify subRangeSet returned by TreeRangeSet.subRangeSet(Range).
IMO, it should be availiable to do so as specified in the document.

```java
import com.google.common.collect.Range;
import com.google.common.collect.TreeRangeSet;
import org.junit.jupiter.api.Test;

@SuppressWarnings("UnstableApiUsage")
public class SubRangeSetTest {

    @Test
    public void testAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.subRangeSet(range).add(range);
    }

    @Test
    public void testReplaceAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.add(range);
        set.subRangeSet(range).add(range);
    }
}
```

```
java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)
	at com.google.common.collect.TreeRangeSet.replaceRangeWithSameLowerBound(TreeRangeSet.java:266)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:216)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testAdd(SubRangeSetTest.java:12)
```
```
java.lang.UnsupportedOperationException
	at com.google.common.collect.UnmodifiableIterator.remove(UnmodifiableIterator.java:46)
	at com.google.common.collect.Iterators.clear(Iterators.java:985)
	at com.google.common.collect.Maps$IteratorBasedAbstractMap.clear(Maps.java:3562)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:214)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testReplaceAdd(SubRangeSetTest.java:20)
```
public void add(Range<C> rangeToAdd) {
    checkNotNull(rangeToAdd);
    if (rangeToAdd.isEmpty()) {
      return;
    }
    
    
    Cut<C> lbToAdd = rangeToAdd.lowerBound;
    Cut<C> ubToAdd = rangeToAdd.upperBound;
    Entry<Cut<C>, Range<C>> entryBelowLB = rangesByLowerBound.lowerEntry(lbToAdd);
    if (entryBelowLB != null) {
      
      Range<C> rangeBelowLB = entryBelowLB.getValue();
      if (rangeBelowLB.upperBound.compareTo(lbToAdd) >= 0) {
        
        if (rangeBelowLB.upperBound.compareTo(ubToAdd) >= 0) {
          
          ubToAdd = rangeBelowLB.upperBound;
          /*
           * TODO(cpovirk): can we just "return;" here? Or, can we remove this if() entirely? If
           * not, add tests to demonstrate the problem with each approach
           */
        }
        lbToAdd = rangeBelowLB.lowerBound;
      }
    }
    Entry<Cut<C>, Range<C>> entryBelowUB = rangesByLowerBound.floorEntry(ubToAdd);
    if (entryBelowUB != null) {
      
      Range<C> rangeBelowUB = entryBelowUB.getValue();