Compiler assert: $defined of generic type

```c
fn void main() {
  $defined(Foo{int});
}
```
`⚠️ The compiler encountered an unexpected error: "Violated assert: decl->decl_kind == DECL_FUNC".`

Bonus error:
```c
fn void main() {
  $defined(Foo{int}****); // Error: An expression was expected
}
```