[Ada] Small cleanup in Apply_Range_Check implementation

[Ada] Small cleanup in Apply_Range_Check implementation

2020-06-19  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* checks.ads (Apply_Static_Length_Check): Move up.
	(Apply_Range_Check): Add parameter Insert_Node.
	* checks.adb (Apply_Selected_Range_Checks): Merge into...
	(Apply_Range_Check): ...this.  Add parameter Insert_Node,
	pass it as Warn_Node to Selected_Range_Checks and use it
	as insertion point for the checks.
	* sem_ch3.adb (Analyze_Subtype_Declaration): Rewrite block
	dealing with the range checks for the subtype indication.
	Use local variable and call Apply_Range_Check in both cases.