SaveChangesAsync should create savepoints (and roll back to them) when under an ambient transaction

SaveChangesAsync currently manages transaction savepoints when it's executed within a user-managed transaction. However, it does not create a savepoint if an ambient System.Transactions transaction is in progress. We already have some detection and special handling of ambient transactions, it probably makes sense to have savepoints for this case as well.

Originally raised by @davidroth in #36470