[ty] Type narrowing in comprehensions (#18934)

## Summary

Add type narrowing inside comprehensions:

```py
def _(xs: list[int | None]):
    [reveal_type(x) for x in xs if x is not None]  # revealed: int
```

closes https://github.com/astral-sh/ty/issues/680

## Test Plan

* New Markdown tests
* Made sure the example from https://github.com/astral-sh/ty/issues/680
now checks without errors
* Made sure that all removed ecosystem diagnostics were actually false
positives

crates/ty_python_semantic/src/semantic_index/builder.rs

@@ -854,8 +854,9 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
             value,
         );
 
-        for expr in &generator.ifs {
-            self.visit_expr(expr);
+        for if_expr in &generator.ifs {
+            self.visit_expr(if_expr);
+            self.record_expression_narrowing_constraint(if_expr);
         }
 
         for generator in generators_iter {
@@ -871,8 +872,9 @@ impl<'db, 'ast> SemanticIndexBuilder<'db, 'ast> {
                 value,
             );
 
-            for expr in &generator.ifs {
-                self.visit_expr(expr);
+            for if_expr in &generator.ifs {
+                self.visit_expr(if_expr);
+                self.record_expression_narrowing_constraint(if_expr);
             }
         }
 

crates/ty_python_semantic/src/types/infer.rs

@@ -5089,7 +5089,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             .iterate(builder.db())
         });
         for expr in ifs {
-            self.infer_expression(expr);
+            self.infer_standalone_expression(expr);
         }
     }