[Ada] Correct time zone in GNAT.Calendar.Time_IO.Value

[Ada] Correct time zone in GNAT.Calendar.Time_IO.Value

gcc/ada/

	* libgnat/g-catiio.adb (Parse_ISO_8601): New name for
	Parse_ISO_8861_UTC.  8601 is the correct ISO standard number.
	Also, "UTC" was confusing. All Time values are represented in
	UTC, but the ISO 8601 date strings include a time zone.

	If a time zone was specified, call
	Ada.Calendar.Formatting.Time_Of instead of
	GNAT.Calendar.Time_Of, because the latter adjusts to the current
	time zone, whereas we want to use (just) the time zone specified
	in the ISO string.  This allows us to pass Time_Zone instead to
	Time_Of, instead of adjusting by Local_Disp by hand.

	If no time zone was specified, call GNAT.Calendar.Time_Of as
	before.

	Use expanded names to clarify which Time_Of is being called.
	Remove redundant comment, and move nonredundant part of the
	commment to the spec.
	(Value): Minor: use "not in" instead of "or else".
	* libgnat/g-catiio.ads: Comment moved here. Correct the ISO
	standard number.
	* libgnat/g-calend.adb: Add ??? comments.
	* libgnat/a-calend.ads, libgnat/a-calend.adb: Update obsolete
	comments regarding the representation of type Time. Move the
	information about the epoch (year 2150) to the spec, and avoid
	uttering "2150" more than once.
	* libgnat/a-catizo.ads (Time_Offset): Add comment.