This is re: hipstersmoothie/react-docgen-typescript-plugin#51 and storybookjs/storybook#15401 cc @hipstersmoothie @leepowelldev

tl;dr This PR would allow consumers who parse components with react-docgen-typescript to access the expression for that component.

Use case

In Storybook, a component's prop table will break if you set the component's displayName to a value that differs from its identifier. For example, let's say that you define a component like this:

export const Button = (props: ButtonComponentProps) => (
  <button>{props.text}</button>
);

Button.displayName = "MyButtonDisplayName";

Then react-docgen-typescript-plugin will append code to that module like this:

try {
    // @ts-ignore
    MyButtonDisplayName.displayName = "MyButtonDisplayName";
    // @ts-ignore
    MyButtonDisplayName.__docgenInfo = { "description": "", "displayName": "MyButtonDisplayName", "props": { "text": { "defaultValue": null, "description": "", "name": "text", "required": true, "type": { "name": "string" } } } };
}
catch (__react_docgen_typescript_loader_error) { }

That code will be a no-op, because MyButtonDisplayName is an invalid reference.

Why does react-docgen-typescript-plugin behave this way? Because the react-docgen-typescript parser doesn't return any way of obtaining the component's identifier other than displayName.

If the parser returned the component expression, then consumers like react-docgen-typescript-plugin could use expression.getName() instead of displayName to correctly handle cases where displayName is a different value.

Note that I opted to return the expression object here, rather than just the result of expression.getName(), because it might help to address another case: unnamed default exports. react-docgen-typescript-plugin fails for component declarations of the form export default function() { ... }, since there's no identifier to reference the component in that case. But using the expression, it might be able to inject one—that is, it could convert the component declaration to one of the form export default function SomeGeneratedName() { ... }, making it referencable.