Don't implement AsyncFn for FnDef/FnPtr that wouldnt implement Fn

tests/ui/async-await/async-closures/fn-exception-target-features.rs

@@ -0,0 +1,17 @@
+//@ edition: 2021
+//@ only-x86_64
+
+#![feature(async_closure, target_feature_11)]
+// `target_feature_11` just to test safe functions w/ target features.
+
+use std::pin::Pin;
+use std::future::Future;
+
+#[target_feature(enable = "sse2")]
+fn target_feature()  -> Pin<Box<dyn Future<Output = ()> + 'static>> { todo!() }
+
+fn test(f: impl async Fn()) {}
+
+fn main() {
+    test(target_feature); //~ ERROR the trait bound
+}