Allow SmallPtrSet to be used with a std::insert_iterator

Currently, the SmallPtrSet type allows inserting elements but it does
not support inserting elements with a positional hint. The lack of this
signature means that you cannot use SmallPtrSet with
std::insert_iterator or std::inserter(), which makes some code
constructs more awkward. This adds an overload of insert() that can be
used in these scenarios.

The positional hint is unused by SmallPtrSet and the call is equivalent
to calling insert() without a hint.