[Ada] Add "Global => null" contracts to Ada.Calendar routines

[Florian Weimer]

* Pierre-Marie de Rodat:

>     procedure Split
>       (Date    : Time;
>        Year    : out Year_Number;
>        Month   : out Month_Number;
>        Day     : out Day_Number;
> -      Seconds : out Day_Duration);
> +      Seconds : out Day_Duration)
> +   with
> +     Global => null;

Is this really correct?  Doesn't this call UTC_Time_Offset eventually,
via Formatting_Operations.Split with Use_TZ => False?

Thanks,
Florian