Fix issue with propTypes misclassifying props

[drx]

While testing #2110, I found a bug with declaredPropTypes and the no-unused-prop-types rule.

The propTypes module incorrectly detects function parameters inside a class property as props of the class component, which results in this code being considered a warning:

type Props = {
  used: string,
}

class Hello extends React.Component<Props> {
  renderHelper = ({unused}: {unused: string}) => {
    return <div />;
  }
  render() {
    return <div>{this.props.used}</div>;
  }
}

It thinks that unused is an unused prop of Hello.

I'm not a 100% if the fix is the correct fix, but I made markAnnotatedFunctionArgumentsAsDeclared check if the node has a ClassProperty ancestor, and if so, bail. I also added a test to no-unused-prop-types to check for this.

Having these kinds of class properties is arguably an anti-pattern, but it came up as an issue when I was testing legacy code. Worse yet, because declaredPropTypes is an object, those helper properties can overwrite actual props.

[ljharb]

Thanks!