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/resources/mdtest/narrow/match.md

@@ -16,3 +16,48 @@ def _(flag: bool):
 
     reveal_type(y)  # revealed: Literal[0] | None
 ```
+
+## Class patterns
+
+```py
+def get_object() -> object: ...
+
+class A: ...
+class B: ...
+
+x = get_object()
+
+reveal_type(x)  # revealed: object
+
+match x:
+    case A():
+        reveal_type(x)  # revealed: A
+    case B():
+        # TODO could be `B & ~A`
+        reveal_type(x)  # revealed: B
+
+reveal_type(x)  # revealed: object
+```
+
+## Class pattern with guard
+
+```py
+def get_object() -> object: ...
+
+class A:
+    def y() -> int: ...
+
+class B: ...
+
+x = get_object()
+
+reveal_type(x)  # revealed: object
+
+match x:
+    case A() if reveal_type(x):  # revealed: A
+        pass
+    case B() if reveal_type(x):  # revealed: B
+        pass
+
+reveal_type(x)  # revealed: object
+```