Respect async expressions in comprehension bodies (#11219)

## Summary

We weren't recursing into the comprehension body.

crates/ruff_linter/resources/test/fixtures/ruff/RUF029.py

@@ -73,3 +73,8 @@ async def fail_4c():  # RUF029: the /inner/ function does not await
 
 async def test():
     return [check async for check in async_func()]
+
+
+async def test() -> str:
+    vals = [str(val) for val in await async_func(1)]
+    return ",".join(vals)

crates/ruff_linter/src/rules/ruff/rules/unused_async.rs

@@ -58,14 +58,6 @@ impl<'a> preorder::PreorderVisitor<'a> for AsyncExprVisitor {
             preorder::TraversalSignal::Traverse
         }
     }
-    fn visit_expr(&mut self, expr: &'a Expr) {
-        match expr {
-            Expr::Await(_) => {
-                self.found_await_or_async = true;
-            }
-            _ => preorder::walk_expr(self, expr),
-        }
-    }
     fn visit_stmt(&mut self, stmt: &'a Stmt) {
         match stmt {
             Stmt::With(ast::StmtWith { is_async: true, .. }) => {
@@ -84,9 +76,19 @@ impl<'a> preorder::PreorderVisitor<'a> for AsyncExprVisitor {
             _ => preorder::walk_stmt(self, stmt),
         }
     }
+    fn visit_expr(&mut self, expr: &'a Expr) {
+        match expr {
+            Expr::Await(_) => {
+                self.found_await_or_async = true;
+            }
+            _ => preorder::walk_expr(self, expr),
+        }
+    }
     fn visit_comprehension(&mut self, comprehension: &'a ast::Comprehension) {
         if comprehension.is_async {
             self.found_await_or_async = true;
+        } else {
+            preorder::walk_comprehension(self, comprehension);
         }
     }
 }