Remember AdjustementRule index

Remember AdjustementRule index

Profiling TimeZoneInfo scenarios on Unix shows most of the time being spent in GetPreviousAdjustmentRule. This is because we need to scan all of the AdjustmentRules, looking for the specified rule, and then getting the one before it.

This change remembers the AdjustmentRule index when passing the AdjustmentRule into GetPreviousAdjustmentRule. This allows us to not scan all the rules, and increases the performance of DateTime.Now significantly in time zones that have daylight savings time.

Fix https://github.com/dotnet/corefx/issues/3571