This patch adds a new function Compilation_ISO_Date to package
GNAT.Source_Info. It is identical to Compilation_Date, except
that the date is returned in standard ISO form -- "yyyy-mm-dd".
This form has the advantage that string comparisons like "<"
can be used to determine the order of dates.

The following source file should compile with errors
(assuming the current date is in the 21'st century):

gnatmake -q -f very_obsolete.ads

very_obsolete.ads:4:30: Delete this package from library
gnatmake: "very_obsolete.ads" compilation error

with GNAT.Source_Info;
package Very_Obsolete is
   pragma Obsolescent
     (Very_Obsolete,"This package will be compleatly removed  1999-01-01");
   pragma Compile_Time_Error
     (GNAT.Source_Info.Compilation_ISO_Date > "1999-01-01",
      "Delete this package from library");
end Very_Obsolete;

Tested on x86_64-pc-linux-gnu, committed on trunk

2016-04-18  Bob Duff  <duff@adacore.com>

	* g-souinf.ads (Compilation_ISO_Date): New function to return
	the current date in ISO form.
	* exp_intr.adb (Expand_Source_Info, Add_Source_Info): Expand
	a call to Compilation_ISO_Date into a string literal containing
	the current date in ISO form.
	* exp_intr.ads (Add_Source_Info): Improve documentation.
	* sem_intr.adb (Check_Intrinsic_Subprogram): Recognize
	Compilation_ISO_Date.
	* snames.ads-tmpl (Name_Compilation_ISO_Date): New Name_Id.