Provide a better diagnostic on failure to meet send bound on futures in a foreign crate

Adding diagnostic data on generators to the crate metadata and using it to provide
a better diagnostic on failure to meet send bound on futures originated from a foreign crate

src/test/ui/async-await/issues/issue-67893.rs

@@ -7,5 +7,5 @@ fn g(_: impl Send) {}
 
 fn main() {
     g(issue_67893::run())
-    //~^ ERROR generator cannot be sent between threads safely
+    //~^ ERROR future cannot be sent between threads safely
 }

src/test/ui/async-await/issues/issue-67893.stderr

@@ -1,10 +1,22 @@
-error: generator cannot be sent between threads safely
+error: future cannot be sent between threads safely
   --> $DIR/issue-67893.rs:9:7
    |
 LL |     g(issue_67893::run())
-   |       ^^^^^^^^^^^^^^^^^^ generator is not `Send`
+   |       ^^^^^^^^^^^^^^^^^^ future is not `Send`
    |
    = help: within `impl Future<Output = ()>`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
+note: future is not `Send` as this value is used across an await
+  --> $DIR/auxiliary/issue_67893.rs:9:26
+   |
+LL |     f(*x.lock().unwrap()).await;
+   |        ----------------- ^^^^^^ await occurs here, with `x.lock().unwrap()` maybe used later
+   |        |
+   |        has type `MutexGuard<'_, ()>` which is not `Send`
+note: `x.lock().unwrap()` is later dropped here
+  --> $DIR/auxiliary/issue_67893.rs:9:32
+   |
+LL |     f(*x.lock().unwrap()).await;
+   |                                ^
 note: required by a bound in `g`
   --> $DIR/issue-67893.rs:6:14
    |