[Ada] Use Cancel_Special_Output instead of Set_Special_Output with null

I believe Cancel_Special_Output is easier to read and thus preferred;
otherwise, it wouldn't be introduced, so let's use it where possible.

2018-11-14  Piotr Trojanek  <trojanek@adacore.com>

gcc/ada/

	* bindgen.adb, exp_cg.adb, repinfo.adb, sprint.adb: Minor reuse
	Cancel_Special_Output where possible.

From-SVN: r266110

gcc/ada/ChangeLog

 2018-11-14  Piotr Trojanek  <trojanek@adacore.com>
 
+	* bindgen.adb, exp_cg.adb, repinfo.adb, sprint.adb: Minor reuse
+	Cancel_Special_Output where possible.
+
+2018-11-14  Piotr Trojanek  <trojanek@adacore.com>
+
 	* exp_dbug.ads, sem_util.ads: Minor fixes in comments.
 
 2018-11-14  Arnaud Charlet  <charlet@adacore.com>

gcc/ada/bindgen.adb

@@ -1186,7 +1186,7 @@ package body Bindgen is
       end loop;
       WBI ("     & ASCII.NUL;");
 
-      Set_Special_Output (null);
+      Cancel_Special_Output;
 
       Bind_Env_String_Built := True;
    end Gen_Bind_Env_String;

gcc/ada/exp_cg.adb

@@ -146,7 +146,7 @@ package body Exp_CG is
          end if;
       end loop;
 
-      Set_Special_Output (null);
+      Cancel_Special_Output;
    end Generate_CG_Output;
 
    ----------------

gcc/ada/repinfo.adb

@@ -1756,7 +1756,7 @@ package body Repinfo is
                     (Get_Name_String (File_Name (Source_Index (U))));
                   Set_Special_Output (Write_Info_Line'Access);
                   List_Entities (Cunit_Entity (U), Bytes_Big_Endian);
-                  Set_Special_Output (null);
+                  Cancel_Special_Output;
                   Close_Repinfo_File_Access.all;
                end if;
             end if;

gcc/ada/sprint.adb

@@ -660,7 +660,7 @@ package body Sprint is
                   Write_Source_Lines (Last_Source_Line (Current_Source_File));
                   Write_Eol;
                   Close_Debug_Source;
-                  Set_Special_Output (null);
+                  Cancel_Special_Output;
 
                --  Normal output to standard output file