[Ada] Use Standard.Natural on indices in support routines for Ada.Tags

[Ada] Use Standard.Natural on indices in support routines for Ada.Tags

2020-06-02  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_atag.ads (Build_Inherit_Predefined_Prims): Change type
	of Num_Predef_Prim parameter from Int to Nat.
	* exp_atag.adb (Build_Range): New procedure.
	(Build_Val): Likewise.
	(Build_CW_Membership): Call Build_Val.
	(Build_Get_Predefined_Prim_Op_Address): Likewise.
	(Build_Inherit_CPP_Prims): Likewise.
	(Build_Get_Prim_Op_Address): Likewise.
	(Build_Set_Predefined_Prim_Op_Address): Likewise.
	(Build_Inherit_Prims): Call Build_Range.
	(Build_Inherit_Predefined_Prims): Likewise.  Change type of
	Num_Predef_Prim parameter from Int to Nat.