[Ada] Improve messages on incorrect state refinement in SPARK

[Ada] Improve messages on incorrect state refinement in SPARK

gcc/ada/

	* sem_ch10.adb (Is_Private_Library_Unit): Move query to
	Sem_Util for sharing.
	* sem_ch7.adb (Analyze_Package_Body_Helper): Add continuation
	message.
	* sem_prag.adb (Analyze_Part_Of): Call new function
	Is_Private_Library_Unit.
	(Check_Valid_Library_Unit_Pragma): Specialize error messages on
	misplaced pragmas.
	(Analyze_Refined_State_In_Decl_Part): Recognize missing Part_Of
	on object in private part.
	* sem_util.adb (Check_State_Refinements): Add continuation
	message.
	(Find_Placement_In_State_Space): Fix detection of placement,
	which relied wrongly on queries In_Package_Body/In_Private_Part
	which do not provide the right information here for all cases.
	(Is_Private_Library_Unit): Move query here for sharing.
	* sem_util.ads (Is_Private_Library_Unit): Move query here for
	sharing.