Add error for using generalized expressions with export assignments in ambient contexts

[weswigham]

Fixes #17991

[mhegazy]

Please add an entry for this change in https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#typescript-26

[weswigham]

@mhegazy done.

[ghost]

@weswigham I thought we were going to allow export = class? DefinitelyTyped/types/express-brute-mongo is broken by this.

[weswigham]

@andy-ms Nope, we said identifier or qualified name only. export default class gets a pass, since it gets parsed as a modifier on a class statement (rather than a default exported class expression) (and causes appropriate grammatical errors on the class from an ambient context); export =, however, always parses as an expression; ergo: only identifiers or qualified names allowed in ambient contexts.
cc @mhegazy for feedback.

@andy-ms, can it be rewritten as

import { Collection } from "mongodb";

/**
 * @summary MongoDB store adapter.
 * @class
 */
class MongoStore {
    /**
     * @summary Constructor.
     * @constructor
     * @param {Function} getCollection The collection.
     * @param {Object}   options       The otpions.
     */
    constructor(getCollection: (collection: (c: Collection) => void) => void, options?: Object);
}
export = MongoStore;

?