Make async gen fn an error

tests/ui/coroutine/async_gen_fn.rs

@@ -0,0 +1,11 @@
+// edition: 2024
+// compile-flags: -Zunstable-options
+#![feature(gen_blocks)]
+
+// async generators are not yet supported, so this test makes sure they make some kind of reasonable
+// error.
+
+async gen fn foo() {}
+//~^ `async gen` functions are not supported
+
+fn main() {}