[red-knot] Add new 'unreachable code' test case (#17306)

## Summary

This is a new test case that I don't know how to handle yet. It leads to
many false positives in `rich/tests/test_win32_console.py`, which does
something like:

```py
if sys.platform == "win32":
    from windows_only_module import some_symbol

    some_other_symbol = 1

    def some_test_case():
        use(some_symbol)  # Red Knot: unresolved-reference
        use(some_other_symbol)  # Red Knot: unresolved-reference
```

Also adds a test for using unreachable symbols in type annotations or as
class bases.

crates/red_knot_python_semantic/resources/mdtest/unreachable.md

@@ -432,6 +432,44 @@ if sys.version_info >= (3, 11):
     import wsgiref.types
 ```
 
+### Nested scopes
+
+When we have nested scopes inside the unreachable section, we should not emit diagnostics either:
+
+```py
+if False:
+    x = 1
+
+    def f():
+        # TODO
+        # error: [unresolved-reference]
+        print(x)
+
+    class C:
+        def __init__(self):
+            # TODO
+            # error: [unresolved-reference]
+            print(x)
+```
+
+### Use of unreachable symbols in type annotations, or as class bases
+
+We should not show any diagnostics in type annotations inside unreachable sections.
+
+```py
+def _():
+    class C: ...
+    return
+
+    # TODO
+    # error: [invalid-type-form] "Variable of type `Never` is not allowed in a type expression"
+    c: C = C()
+
+    # TODO
+    # error: [invalid-base] "Invalid class base with type `Never` (all bases must be a class, `Any`, `Unknown` or `Todo`)"
+    class Sub(C): ...
+```
+
 ### Emit diagnostics for definitely wrong code
 
 Even though the expressions in the snippet below are unreachable, we still emit diagnostics for