Add support to ClassExpressions in the prop-types rule

[jseminck]

I'm not super happy with the solution, perhaps others have a better idea?

It turns out that ESLint visits ClassExpression before it visits TypeParameterDeclaration, but I want it the other way around.

I tried using the ClassExpression:exit key, but it doesn't change anything.

Therefore, I needed a way to defer class expressions evaluations util after TypeParameterDeclaration has been visited, by storing the nodes in an array and looping through it in Program:exit.

This fixes #1376 (comment) and several other cases which we didn't catch before.

I'm not sure it will cover all of the issues related to Flow Props yet.

I'll add the same support to no-unused-prop-types in #1393

[ljharb]

This seems like the proper approach to me

[johnhaley81]

@jseminck I just tested this out in my code and it's working great!