Rename in preparation for moving the `async` printing out of `CoroutineSource`

compiler/rustc_hir/src/hir.rs

@@ -1568,7 +1568,7 @@ impl CoroutineSource {
         match self {
             CoroutineSource::Block => "`async` block",
             CoroutineSource::Closure => "`async` closure body",
-            CoroutineSource::Fn => "`async fn` body",
+            CoroutineSource::Fn => "`async` fn body",
         }
     }
 }

tests/ui/async-await/async-await-let-else.stderr

@@ -30,7 +30,7 @@ LL |     is_send(foo2(Some(true)));
    |     required by a bound introduced by this call
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `Rc<()>`
-note: required because it's used within this `async fn` body
+note: required because it's used within this `async` fn body
   --> $DIR/async-await-let-else.rs:24:29
    |
 LL |   async fn bar2<T>(_: T) -> ! {
@@ -39,7 +39,7 @@ LL | |     panic!()
 LL | | }
    | |_^
    = note: required because it captures the following types: `impl Future<Output = !>`
-note: required because it's used within this `async fn` body
+note: required because it's used within this `async` fn body
   --> $DIR/async-await-let-else.rs:18:32
    |
 LL |   async fn foo2(x: Option<bool>) {

tests/ui/async-await/issue-68112.stderr

@@ -45,7 +45,7 @@ LL |     require_send(send_fut);
    = help: the trait `Sync` is not implemented for `RefCell<i32>`
    = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock` instead
    = note: required for `Arc<RefCell<i32>>` to implement `Send`
-note: required because it's used within this `async fn` body
+note: required because it's used within this `async` fn body
   --> $DIR/issue-68112.rs:47:31
    |
 LL |   async fn ready2<T>(t: T) -> T {

tests/ui/async-await/issue-70935-complex-spans.stderr

@@ -18,7 +18,7 @@ note: required because it's used within this closure
    |
 LL |         baz(|| async {
    |             ^^
-note: required because it's used within this `async fn` body
+note: required because it's used within this `async` fn body
   --> $DIR/issue-70935-complex-spans.rs:12:67
    |
 LL |   async fn baz<T>(_c: impl FnMut() -> T) where T: Future<Output=()> {

tests/ui/async-await/issues/issue-67893.stderr

@@ -13,7 +13,7 @@ LL | pub async fn run() {
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
    = note: required because it captures the following types: `Arc<Mutex<()>>`, `MutexGuard<'_, ()>`, `impl Future<Output = ()>`
-note: required because it's used within this `async fn` body
+note: required because it's used within this `async` fn body
   --> $DIR/auxiliary/issue_67893.rs:9:20
    |
 LL |   pub async fn run() {

tests/ui/async-await/partial-drop-partial-reinit.rs

@@ -26,7 +26,7 @@ impl Drop for NotSend {
 impl !Send for NotSend {}
 
 async fn foo() {
-    //~^ NOTE used within this `async fn` body
+    //~^ NOTE used within this `async` fn body
     //~| NOTE within this `impl Future
     let mut x = (NotSend {},);
     drop(x.0);

tests/ui/async-await/partial-drop-partial-reinit.stderr

@@ -12,7 +12,7 @@ LL | async fn foo() {
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `NotSend`
    = note: required because it appears within the type `(NotSend,)`
    = note: required because it captures the following types: `(NotSend,)`, `impl Future<Output = ()>`
-note: required because it's used within this `async fn` body
+note: required because it's used within this `async` fn body
   --> $DIR/partial-drop-partial-reinit.rs:28:16
    |
 LL |   async fn foo() {