for-each loop over BSONObj is slower than explicit BSONObjIterator in MatchExpressionParser code

As a part of SERVER-30761, we refactored parseInExpression() and changed a loop that looked like

BSONObjIterator iter(theArray);

while (iter.more()) {

    auto e = iter.next();

    // ...

}

into

for (auto e : theArray) {

    // ...

}

This has caused a performance regression of roughly 5% when parsing a $in, and is apparent with very large in expressions. The real fix seems like achieving performance parity between the two ways to iterate through a BSONObj, but I'm not sure how much we want to dive down the rabbit hole in terms of seeing the machine code that's generated by the two constructs.