[red-knot] fix ordering of ClassDef semantic index building (#16915)

## Summary

From #16861

This PR fixes the incorrect `ClassDef` handling of
`SemanticIndexBuilder::visit_stmt`, which fixes some of the incorrect
behavior of referencing the class itself in the class scope (a complete
fix requires a different fix, which will be done in the another PR).

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -926,9 +926,6 @@ where
                     self.visit_decorator(decorator);
                 }
 
-                let symbol = self.add_symbol(class.name.id.clone());
-                self.add_definition(symbol, class);
-
                 self.with_type_params(
                     NodeWithScopeRef::ClassTypeParameters(class),
                     class.type_params.as_deref(),
@@ -943,6 +940,10 @@ where
                         builder.pop_scope()
                     },
                 );
+
+                // In Python runtime semantics, a class is registered after its scope is evaluated.
+                let symbol = self.add_symbol(class.name.id.clone());
+                self.add_definition(symbol, class);
             }
             ast::Stmt::TypeAlias(type_alias) => {
                 let symbol = self.add_symbol(