Add a binding kind for comprehension targets (#9967)

## Summary

I was surprised to learn that we treat `x` in `[_ for x in y]` as an
"assignment" binding kind, rather than a dedicated comprehension
variable.

crates/ruff_python_semantic/src/binding.rs

@@ -432,6 +432,12 @@ pub enum BindingKind<'a> {
     /// ```
     LoopVar,
 
+    /// A binding for a comprehension variable, like `x` in:
+    /// ```python
+    /// [x for x in range(10)]
+    /// ```
+    ComprehensionVar,
+
     /// A binding for a with statement variable, like `x` in:
     /// ```python
     /// with open('foo.py') as x: