Error on using yield without also using #[coroutine] on the closure

And suggest adding the `#[coroutine]` to the closure

tests/source/immovable_coroutines.rs

@@ -1,7 +1,8 @@
 #![feature(coroutines)]
 
 unsafe fn foo() {
-    let mut ga = static || { 
+    let mut ga = #[coroutine]
+    static || {
         yield 1;
     };
 }

tests/target/immovable_coroutines.rs

@@ -1,7 +1,8 @@
 #![feature(coroutines)]
 
 unsafe fn foo() {
-    let mut ga = static || {
+    let mut ga = #[coroutine]
+    static || {
         yield 1;
     };
 }