Dont ICE when computing coverage of synthetic async closure body

tests/coverage/async_closure.rs

@@ -0,0 +1,15 @@
+#![feature(async_closure)]
+//@ edition: 2021
+
+//@ aux-build: executor.rs
+extern crate executor;
+
+async fn call_once(f: impl async FnOnce()) {
+    f().await;
+}
+
+pub fn main() {
+    let async_closure = async || {};
+    executor::block_on(async_closure());
+    executor::block_on(call_once(async_closure));
+}