[Utils] Hide the default behavior change of D68819 under a flag

With D69701, the options used when running the script on a file will be
recorded and reused on a rerun. This allows us to hide new features
behind flags, starting with the "define" that was introduced in D68819.
diff --git a/llvm/utils/update_test_checks.py b/llvm/utils/update_test_checks.py
index 6cb302b..97bf6e9 100755
--- a/llvm/utils/update_test_checks.py
+++ b/llvm/utils/update_test_checks.py
@@ -180,7 +180,7 @@
 
         # Print out the various check lines here.
         common.add_ir_checks(output_lines, ';', prefix_list, func_dict,
-                             func_name, args.preserve_names)
+                             func_name, args.preserve_names, args.function_signature)
         is_in_function_start = False
 
       if is_in_function: