Fix PyCLIF-C-API codegen bug for @extend @classmethod (b/334753945).

Support for `@extend` `@classmethod` was added with cl/331177628. The bug fixed by this CL was introduced with cl/333112933 in Sep 2020.

Manual testing using this command:

```
blaze test --config=asan third_party/clif/testing/python:extend_classmethods_test
```

* With the new test code but WITHOUT the change in clif/python/gen.py: http://sponge2/c9071f9f-d4df-4e42-88a8-02619b814418 (failed)

```
SUMMARY: UndefinedBehaviorSanitizer: signed-integer-overflow third_party/clif/testing/python/extend_classmethods_clif_aux.h:22:29
```

* With this CL as mailed: http://sponge2/4e335657-ff83-4300-ae4d-0b68003346e5 (passed)

* With `--//devtools/clif/python/codegen_mode=pybind11`: http://sponge2/f765d23c-caed-4402-921c-eae64b122068 (passed)

PiperOrigin-RevId: 625564244

clif/python/gen.py

@@ -878,8 +878,8 @@ def YieldCheckNullptr(ii):
       else:
         num_params = n
         # extended methods need to include `self` as the first parameter, but
-        # extended constructors do not.
-        if func_ast.is_extend_method:
+        # extended classmethods (and constructors) do not.
+        if func_ast.is_extend_method and not func_ast.classmethod:
           num_params += 1
         call_with_params = call + astutils.TupleStr(params[:num_params])
       yield I+I+'%s; break;' % call_with_params

clif/testing/python/extend_classmethods.clif

@@ -29,6 +29,12 @@ from "clif/testing/python/extend_classmethods_clif_aux.h":
       @classmethod
       def set_static_value(cls, v: int)
 
+      @extend
+      @classmethod
+      def function_with_defaults(cls, i: int = default,
+                                      j: int = default,
+                                      k: int = default) -> int
+
     class TestNestedClassmethod:
       class Inner:
         @extend

clif/testing/python/extend_classmethods_clif_aux.h

@@ -17,6 +17,11 @@ inline void Abc__extend__set_static_value(int v) {
   Abc::static_value = v;
 }
 
+inline int Abc__extend__function_with_defaults(int i = 3, int j = 5,
+                                               int k = 7) {
+  return (Abc::static_value * i + j) * k;
+}
+
 inline int TestNestedClassmethod_Inner__extend__get_static_value() {
   return 472;
 }

clif/testing/python/extend_classmethods_test.py

@@ -19,15 +19,27 @@
 
 class ExtendClassmethodsTest(absltest.TestCase):
 
+  def setUp(self):
+    super().setUp()
+    extend_classmethods.Abc.set_static_value(5432)
+
   def test_create_from_value(self):
     expected_value = 543
     abc = extend_classmethods.Abc.from_value(expected_value)
     self.assertEqual(abc.get_value(), expected_value)
 
   def test_set_class_variable(self):
-    expected_value = 5432
-    extend_classmethods.Abc.set_static_value(expected_value)
-    self.assertEqual(extend_classmethods.Abc.get_static_value(), expected_value)
+    self.assertEqual(extend_classmethods.Abc.get_static_value(), 5432)
+
+  def test_function_with_defaults(self):
+    fwd = extend_classmethods.Abc.function_with_defaults
+    self.assertEqual(fwd(), (5432 * 3 + 5) * 7)
+    self.assertEqual(fwd(2), (5432 * 2 + 5) * 7)
+    self.assertEqual(fwd(2, 6), (5432 * 2 + 6) * 7)
+    self.assertEqual(fwd(2, 6, 8), (5432 * 2 + 6) * 8)
+    self.assertEqual(fwd(i=11), (5432 * 11 + 5) * 7)
+    self.assertEqual(fwd(j=13), (5432 * 3 + 13) * 7)
+    self.assertEqual(fwd(k=15), (5432 * 3 + 5) * 15)
 
   def test_nested_classmethod(self):
     v = extend_classmethods.TestNestedClassmethod.Inner.get_static_value()