Preserve generator and yield source for error messages

Previously, error messages after HIR lowering all referred
to generators and yield, regardless of whether the original
source was a generator or an async/await body. This change
tracks the kind of each generator and yield source in order
to provide appropriately tailored error messages.

src/test/ui/async-await/unresolved_type_param.rs

@@ -7,9 +7,9 @@ async fn bar<T>() -> () {}
 
 async fn foo() {
     bar().await;
-    //~^ ERROR type inside generator must be known in this context
+    //~^ ERROR type inside `async` object must be known in this context
     //~| NOTE cannot infer type for `T`
-    //~| NOTE the type is part of the generator because of this `yield`
+    //~| NOTE the type is part of the `async` object because of this `await`
     //~| NOTE in this expansion of desugaring of `await`
 }
 fn main() {}

src/test/ui/async-await/unresolved_type_param.stderr

@@ -1,10 +1,10 @@
-error[E0698]: type inside generator must be known in this context
+error[E0698]: type inside `async` object must be known in this context
   --> $DIR/unresolved_type_param.rs:9:5
    |
 LL |     bar().await;
    |     ^^^ cannot infer type for `T`
    |
-note: the type is part of the generator because of this `yield`
+note: the type is part of the `async` object because of this `await`
   --> $DIR/unresolved_type_param.rs:9:5
    |
 LL |     bar().await;