Diagnostic when reading a required input from a constructor/initializer

There are circumstances where we can know statically that a required input is accessed in a context where it will not have a value:

```ts
id = input.required<number>();
data$ = httpClient.get('/api/user/' + this.id()); // id() read is statically in ctor scope
```

This is guaranteed to throw a runtime error. We can also produce a compiler diagnostic in this case.