gh-115999: Specialize STORE_ATTR in free-threaded builds. (gh-127838)

* Add `_PyDictKeys_StringLookupSplit` which does locking on dict keys and
  use in place of `_PyDictKeys_StringLookup`.

* Change `_PyObject_TryGetInstanceAttribute` to use that function
  in the case of split keys.

* Add `unicodekeys_lookup_split` helper which allows code sharing
  between `_Py_dict_lookup` and `_PyDictKeys_StringLookupSplit`.

* Fix locking for `STORE_ATTR_INSTANCE_VALUE`.  Create
  `_GUARD_TYPE_VERSION_AND_LOCK` uop so that object stays locked and
  `tp_version_tag` cannot change.

* Pass `tp_version_tag` to `specialize_dict_access()`, ensuring
  the version we store on the cache is the correct one (in case of
  it changing during the specalize analysis).

* Split `analyze_descriptor` into `analyze_descriptor_load` and
  `analyze_descriptor_store` since those don't share much logic.
  Add `descriptor_is_class` helper function.

* In `specialize_dict_access`, double check `_PyObject_GetManagedDict()`
  in case we race and dict was materialized before the lock.

* Avoid borrowed references in `_Py_Specialize_StoreAttr()`.

* Use `specialize()` and `unspecialize()` helpers.

* Add unit tests to ensure specializing happens as expected in FT builds.

* Add unit tests to attempt to trigger data races (useful for running under TSAN).

* Add `has_split_table` function to `_testinternalcapi`.

Lib/test/test_opcache.py

@@ -1383,6 +1383,72 @@ class TestSpecializer(TestBase):
         self.assert_specialized(send_yield_from, "SEND_GEN")
         self.assert_no_opcode(send_yield_from, "SEND")
 
+    @cpython_only
+    @requires_specialization_ft
+    def test_store_attr_slot(self):
+        class C:
+            __slots__ = ['x']
+
+        def set_slot():
+            c = C()
+            for i in range(100):
+                c.x = i
+
+        set_slot()
+
+        self.assert_specialized(set_slot, "STORE_ATTR_SLOT")
+        self.assert_no_opcode(set_slot, "STORE_ATTR")
+
+        # Adding a property for 'x' should unspecialize it.
+        C.x = property(lambda self: None, lambda self, x: None)
+        set_slot()
+        self.assert_no_opcode(set_slot, "STORE_ATTR_SLOT")
+
+    @cpython_only
+    @requires_specialization_ft
+    def test_store_attr_instance_value(self):
+        class C:
+            pass
+
+        def set_value():
+            c = C()
+            for i in range(100):
+                c.x = i
+
+        set_value()
+
+        self.assert_specialized(set_value, "STORE_ATTR_INSTANCE_VALUE")
+        self.assert_no_opcode(set_value, "STORE_ATTR")
+
+        # Adding a property for 'x' should unspecialize it.
+        C.x = property(lambda self: None, lambda self, x: None)
+        set_value()
+        self.assert_no_opcode(set_value, "STORE_ATTR_INSTANCE_VALUE")
+
+    @cpython_only
+    @requires_specialization_ft
+    def test_store_attr_with_hint(self):
+        class C:
+            pass
+
+        c = C()
+        for i in range(29):
+            setattr(c, f"_{i}", None)
+
+        def set_value():
+            for i in range(100):
+                c.x = i
+
+        set_value()
+
+        self.assert_specialized(set_value, "STORE_ATTR_WITH_HINT")
+        self.assert_no_opcode(set_value, "STORE_ATTR")
+
+        # Adding a property for 'x' should unspecialize it.
+        C.x = property(lambda self: None, lambda self, x: None)
+        set_value()
+        self.assert_no_opcode(set_value, "STORE_ATTR_WITH_HINT")
+
     @cpython_only
     @requires_specialization_ft
     def test_to_bool(self):