Unidempotency with comment + return + , expression

[bakkot]

After #2388 (which hasn't landed as of this writing), there's a commented out test in tests/comments/return-statement.js, reading

Playground

function sequenceExpression() {
  return (
    // Reason for a
    a
  ), b
}

It is commented out because the first time it's formatted it becomes

function sequenceExpression() {
  return (
    // Reason for a
    a,
    b
  );
}

and formatting that gives

function sequenceExpression() {
  return (
    // Reason for a
    a, b
  );
}

We decided this was rare enough not to be worth blocking the PR, but it would be nice to resolve.