react/no-render-return-value should fire for assignment expressions

[jichu4n]

The react/no-render-return-value plugin currently fires for the following:

const x = ReactDOM.render(...);

However, it doesn't fire for the following:

this.x = ReactDOM.render(...);

or the following:

let x;
x = ReactDOM.render(...)

In these cases the parent expression type of the ReactDOM.render() call is AssignmentExpression, which is not in the list of expression types blacklisted by the plugin (source). This is rather surprising behavior.

[jichu4n]

Sent PR #2462 to catch these cases.