Compiler assert: Casting bitstruct to short/char

```c
bitstruct Foo : ushort {
  bool bar;
}

fn void main() {
  ushort foo = (ushort)((Foo) {.bar}) & 1;
}
```
```
⚠️ The compiler encountered an unexpected error: "Violated assert: expr->const_expr.const_kind == CONST_INTEGER || expr->const_expr.const_kind == CONST_ENUM".
```