[ty] Support declaration-only attributes (#19048)

## Summary

Following ty issue [#698](https://github.com/astral-sh/ty/issues/698)
this PR adds support for declarations.

closes #698

## Test Plan

Tested against mdtest (specifically attributes).

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/ty_python_semantic/src/semantic_index/use_def.rs

@@ -509,6 +509,18 @@ impl<'db> UseDefMap<'db> {
             .is_always_false()
     }
 
+    pub(crate) fn is_declaration_reachable(
+        &self,
+        db: &dyn crate::Db,
+        declaration: &DeclarationWithConstraint<'db>,
+    ) -> Truthiness {
+        self.reachability_constraints.evaluate(
+            db,
+            &self.predicates,
+            declaration.reachability_constraint,
+        )
+    }
+
     pub(crate) fn is_binding_reachable(
         &self,
         db: &dyn crate::Db,