Rollup merge of #70011 - petrochenkov:asyncice, r=Centril

def_collector: Fully visit async functions

We forgot to visit attributes previously, it caused ICEs.

Special treatment of async functions is also moved from `visit_item` to `visit_fn` to reuse more of the default visitor.

Fixes https://github.com/rust-lang/rust/issues/67778.

src/test/ui/async-await/expansion-in-attrs.rs

@@ -0,0 +1,13 @@
+// check-pass
+// edition:2018
+
+macro_rules! with_doc {
+    ($doc: expr) => {
+        #[doc = $doc]
+        async fn f() {}
+    };
+}
+
+with_doc!(concat!(""));
+
+fn main() {}