Narrowing for class patterns in match statements (#15223)

We now support class patterns in a match statement, adding a narrowing
constraint that within the body of that match arm, we can assume that
the subject is an instance of that class.

---------

Co-authored-by: Carl Meyer <carl@astral.sh>
Co-authored-by: Micha Reiser <micha@reiser.io>

crates/red_knot_python_semantic/src/visibility_constraints.rs

@@ -329,9 +329,9 @@ impl<'db> VisibilityConstraints<'db> {
                         Truthiness::Ambiguous
                     }
                 }
-                PatternConstraintKind::Singleton(..) | PatternConstraintKind::Unsupported => {
-                    Truthiness::Ambiguous
-                }
+                PatternConstraintKind::Singleton(..)
+                | PatternConstraintKind::Class(..)
+                | PatternConstraintKind::Unsupported => Truthiness::Ambiguous,
             },
         }
     }