[Ada] Refine type of a counter object from Int to Nat

[Ada] Refine type of a counter object from Int to Nat

gcc/ada/

	* exp_ch7.adb (Build_Finalize_Statements): Refine type of a
	local counter variable.
	* exp_dist.adb (Append_Record_Traversal): Refine type of Counter
	parameter.
	(Add_Process_Element): Likewise.
	(Build_From_Any_Call): Refine type of a local counter variable.
	(Build_From_Any_Function): Likewise.
	(Build_To_Any_Function): Likewise.
	(FA_Rec_Add_Process_Element): Likewise.
	(TA_Append_Record_Traversal): Likewise.
	(TA_Rec_Add_Process_Element): Likewise.