Don't let tee_file own a stream

Right now, tee_file owns the second stream it writes to.  This is done
for the convenience of the users.  In a subsequent patch, this will no
longer be convenient, so this patch moves the responsibility for
ownership to the users of tee_file.

gdb/cli/cli-interp.h

@@ -41,6 +41,7 @@ private:
     ui_file *log;
     ui_file *targ;
     ui_file *targerr;
+    ui_file_up tee_to_delete;
     ui_file_up file_to_delete;
     ui_file_up log_to_delete;
   };