[red-knot] Fix recording of negative visibility constraints (#17731)

## Summary

We were previously recording wrong reachability constraints for negative
branches. Instead of `[cond] AND (NOT [True])` below, we were recording
`[cond] AND (NOT ([cond] AND [True]))`, i.e. we were negating not just
the last predicate, but the `AND`-ed reachability constraint from last
clause. With this fix, we now record the correct constraints for the
example from #17723:

```py
def _(cond: bool):
    if cond:
        # reachability: [cond]
        if True:
            # reachability: [cond] AND [True]
            pass
        else:
            # reachability: [cond] AND (NOT [True])
            x
```

closes #17723 

## Test Plan

* Regression test.
* Verified the ecosystem changes

crates/red_knot_python_semantic/resources/mdtest/unreachable.md

@@ -304,6 +304,22 @@ else:
     pass
 ```
 
+And for nested `if` statements:
+
+```py
+def _(flag: bool):
+    if flag:
+        if sys.version_info >= (3, 11):
+            ExceptionGroup  # no error here
+        else:
+            pass
+
+        if sys.version_info < (3, 11):
+            pass
+        else:
+            ExceptionGroup  # no error here
+```
+
 The same works for ternary expressions:
 
 ```py

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -635,7 +635,8 @@ impl<'db> SemanticIndexBuilder<'db> {
             .current_visibility_constraints_mut()
             .add_atom(predicate_id);
         self.current_use_def_map_mut()
-            .record_reachability_constraint(visibility_constraint)
+            .record_reachability_constraint(visibility_constraint);
+        visibility_constraint
     }
 
     /// Record the negation of a given reachability/visibility constraint.