Make TyS::is_suggestable more structual

src/test/ui/return/return-type.stderr

@@ -1,19 +1,15 @@
 error[E0308]: mismatched types
   --> $DIR/return-type.rs:10:5
    |
+LL | fn bar() {
+   |          - possibly return type missing here?
 LL |     foo(4 as usize)
-   |     ^^^^^^^^^^^^^^^ expected `()`, found struct `S`
+   |     ^^^^^^^^^^^^^^^- help: consider using a semicolon here: `;`
+   |     |
+   |     expected `()`, found struct `S`
    |
    = note: expected unit type `()`
                  found struct `S<usize>`
-help: consider using a semicolon here
-   |
-LL |     foo(4 as usize);
-   |                    +
-help: try adding a return type
-   |
-LL | fn bar() -> S<usize> {
-   |          +++++++++++
 
 error: aborting due to previous error