Stop throwing exception in TimeZoneInfo POSIX parsing (#458)

Stop throwing exception in TimeZoneInfo POSIX parsing (#458)

IsDaylightSavingTime_CasablancaMultiYearDaylightSavings fails on rhel.8

When parsing the tzdata POSIX string that contains an 'n' Julian date, we are currently throwing an exception, and then falling back to a TimeZoneInfo without DST enabled. However, this is a mistake because there are other DST transitions that were read from the tzdata file that are valid and usable. We shouldn't be throwing that information away.

So instead, we now skip the POSIX string if we detect an unsupported 'n' Julian date, and just use the last transition as the AdjustmentRule for all the DateTimes in the future. This way we can still make DST determinations correctly for some DateTimes.

Fix https://github.com/dotnet/corefx/issues/42192