Fix DateTime.Now and CurrentTimeToUtc during DST transition time (#4172)

Fix DateTime.Now and CurrentTimeToUtc during DST transition time (#4172)

* During the ambiguous hour .NET will return the DST version of the time whereas mono is returning the base time. Rather than use the transitions array we look up the Adjustment rules (which are created from the transition rules in the first place).

In addition, CurrentTimeToUtc() also has problems around the transition time.

Added test and fixed another.

Fixes #43805.

* Despite the tutorial on msdn about daylight saving time, .NET interprets the 'ambiguous' hour of DST as being DST rather than standard time which is what mono did.