ada: Cleanup inconsistent iteration over exception handlers

ada: Cleanup inconsistent iteration over exception handlers

When detecting duplicate choices in exception handlers we had
inconsistent pairs of First/Next_Non_Pragma and First_Non_Pragma/Next.
This was harmless, because exception choices don't allow pragmas at all,
e.g.:

   when Program_Error | Constraint_Error | ...; --  pragma not allowed

and exception handlers only allow pragmas to appear as the first item
on the list, e.g.:

   exception
      pragma Inspection_Point;   --  first item on the list of handlers
      when Program_Error =>
         <statements>
      pragma Inspection_Point;   --  last item on the list of statements
      when Constraint_Error =>
         ...

However, it still seems cleaner to have consistent pairs of First/Next
and First_Non_Pragma/Next_Non_Pragma.

gcc/ada/

	* sem_ch11.adb
	(Check_Duplication): Fix inconsistent iteration.
	(Others_Present): Iterate over handlers using First_Non_Pragma and
	Next_Non_Pragma just like in Check_Duplication.