[Ada] Follow-on cleanups for Uint fields

[Ada] Follow-on cleanups for Uint fields

gcc/ada/

	* freeze.adb (Freeze_Fixed_Point_Type): Remove
	previously-inserted test for Uint_0; no longer needed.
	* gen_il-gen.ads: Improve comments.
	* repinfo.adb (Rep_Value): Use Ubool type for B.
	* repinfo.ads (Node_Ref): Use Unegative type.
	(Node_Ref_Or_Val): Document that values of this type can be
	No_Uint.
	* exp_disp.adb (Make_Disp_Requeue_Body): Minor comment fix.
	* sem_ch3.adb: Likewise.
	* sem_ch8.adb: Likewise.
	* sinfo-utils.adb (End_Location): End_Span can never be No_Uint,
	so remove the "if No (L)" test.
	* uintp.adb (Image_String): Use "for ... of" loop.
	* uintp.ads (Unegative): New type for negative integers.  We
	give it a long name (unlike Unat and Upos) because it is rarely
	used.