Provide a clearer error message for semicolons after structs and similar

Right now, writing something like `struct S {};` just produces a parse error:

```
error: expected item, found `;`
 --> src/main.rs:1:12
  |
1 | struct S {};
  |            ^ help: consider removing this semicolon
```

We could make this error message much clearer by recognizing this common error and specifically saying "don't put a semicolon after a struct/union/enum declaration".