`@const` fields should be overrideable in subclasses

The following seems reasonable to me, but currently runs afoul of the const checker.

```js
class Base {}
/** @const {number} */
Base.prototype.foo = 0;

class Derived extends Base {
constructor(/** number */ foo) {
/** @const */
this.foo = foo;
}
}
```

The error can be suppressed with `@suppress {const}` on the field assignment, but it doesn't seem like that should be required.