[red-knot] Type narrow in else clause (#13918)

## Summary

Add support for type narrowing in elif and else scopes as part of
#13694.

## Test Plan

- mdtest
- builder unit test for union negation.

---------

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

crates/red_knot_python_semantic/resources/mdtest/subscript/string.md

@@ -23,7 +23,8 @@ reveal_type(b)  # revealed: Unknown
 ## Function return
 
 ```py
-def int_instance() -> int: ...
+def int_instance() -> int:
+    return 42
 
 a = "abcde"[int_instance()]
 # TODO: Support overloads... Should be `str`