[Ada] Cleanup and modification of unreferenced warnings

[Ada] Cleanup and modification of unreferenced warnings

gcc/ada/

	* comperr.adb (Delete_SCIL_Files): Replace unnecessary
	Unreferenced pragma with specific pragma Warnings.
	* doc/gnat_rm/implementation_defined_pragmas.rst (Unreferenced):
	Add documentation for new behavior.
	* gnat_rm.texi: Regenerate.
	* erroutc.adb (Set_At): Remove useless assignment.
	* exp_ch2.adb (In_Assignment_Context): Deleted.
	(Is_Object_Renaming_Name): Replace calls to Is_LHS with calls to
	Known_To_Be_Assigned.
	(Expand_Current_Value): Replace calls to May_Be_Lvalue with
	calls to Known_To_Be_Assigned.
	(Expand_Entry_Paramter): Replace calls to In_Assignment_Context
	with calls to Known_To_Be_Assigned.
	* exp_ch4.adb (Expand_N_Op_Rem): Remove unnecessary Unreferenced
	pragma.
	* exp_imgv.adb (Build_Enumeration_Image_Tables): Default
	initialize S_N.
	* ghost.adb (Check_Ghost_Policy): Replace call to May_Be_Lvalue
	with call to Known_To_Be_Assigned.
	* lib-xref.adb (Is_On_LHS): Deleted.
	(OK_To_Set_Referenced): Rewrite subprogram to encompass the new
	pragma Unreferenced behavior.
	(Process_Deferred_References): Replace call to Is_LHS with call
	to Known_To_Be_Assigned.
	* libgnarl/s-taasde.adb, libgnarl/s-tasren.adb,
	libgnarl/s-tpobop.adb, libgnat/a-calend.adb,
	libgnat/a-calfor.adb, libgnat/a-cbdlli.adb,
	libgnat/a-cbhama.adb, libgnat/a-cbhase.adb,
	libgnat/a-cbmutr.adb, libgnat/a-cborma.adb,
	libgnat/a-cborse.adb, libgnat/a-cdlili.adb,
	libgnat/a-cfhama.adb, libgnat/a-cforse.adb,
	libgnat/a-cidlli.adb, libgnat/a-cihama.adb,
	libgnat/a-cihase.adb, libgnat/a-cimutr.adb,
	libgnat/a-ciorma.adb, libgnat/a-ciormu.adb,
	libgnat/a-ciorse.adb, libgnat/a-cohama.adb,
	libgnat/a-cohase.adb, libgnat/a-comutr.adb,
	libgnat/a-convec.adb, libgnat/a-coorma.adb,
	libgnat/a-coormu.adb, libgnat/a-coorse.adb,
	libgnat/a-crdlli.adb, libgnat/a-tigeau.adb,
	libgnat/a-wtgeau.adb, libgnat/a-ztgeau.adb,
	libgnat/g-calend.adb, libgnat/g-comlin.adb,
	libgnat/g-expect.adb, libgnat/g-mbflra.adb,
	libgnat/g-spipat.adb, libgnat/s-fatgen.adb,
	libgnat/s-fileio.adb, libgnat/s-os_lib.adb,
	libgnat/s-regpat.adb, libgnat/s-valued.adb,
	libgnat/s-valuer.adb: Remove unnecessary Unreferenced pragmas
	* sem_ch10.adb (Process_Spec_Clauses): Remove useless
	assignments.
	* sem_ch13.adb (Validate_Literal_Aspect): Default initialize I.
	* sem_ch3.adb (Build_Derived_Concurrent_Type): Default
	initialize Corr_Decl.
	* sem_ch8.adb (Undefined): Replace calls to Is_LHS with calls to
	Known_To_Be_Assigned.
	(In_Abstract_View_Pragma): Likewise.
	* sem_eval.adb (Eval_Selected_Component): Replace calls to
	Is_LHS with calls to Known_To_Be_Assigned.
	* sem_res.adb (Init_Component): Replace calls to May_Be_Lvalue
	with calls to Known_To_Be_Assigned.
	* sem_util.adb, sem_util.ads (End_Label_Loc): Default initialize
	Owner.
	(Explain_Limited_Type): Default initialize Expr_Func.
	(Find_Actual): Modified to handle entry families.
	(Is_LHS): Deleted.
	(May_Be_Lvalue): Deleted.
	(Known_To_Be_Assigned): Modified and improved to handle all
	cases.
	* sem_warn.adb (Traverse_Result): Replace calls to May_Be_Lvalue
	with calls to Known_To_Be_Assigned.
	(Check_Ref): Modify error on unreferenced out parameters to take
	into account different warning flags.