Properly handle async blocks and fns in if exprs without else

When encountering a tail expression in the then arm of an `if` expression
without an `else` arm, account for `async fn` and `async` blocks to
suggest `return`ing the value and pointing at the return type of the
`async fn`.

We now also account for AFIT when looking for the return type to point at.

Fix #115405.

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -900,7 +900,7 @@ impl<'a> Parser<'a> {
             // fn foo() -> Foo {
             //     field: value,
             // }
-            info!(?maybe_struct_name, ?self.token);
+            debug!(?maybe_struct_name, ?self.token);
             let mut snapshot = self.create_snapshot_for_diagnostic();
             let path = Path {
                 segments: ThinVec::new(),