thenables not invoked from async arrow functions

[kzc]

• Rollup Version: rollup v2.50.5
• Operating System (or Browser): n/a
• Node Version (if applicable): node v16.1.0

Async arrows were not covered in #4115.

Repro

$ cat async-then.js 
// async function
(async function () {
    return {
        get then() {
            return () => effect(11);
        }
    };
})();
// async arrow with block
(async () => {
    return {
        get then() {
            return () => effect(12);
        }
    };
})();
// async arrow without block
(async () => ({
    get then() {
        return () => effect(13);
    }
}))();
const effects = [], effect = x => effects.push(x);
Promise.resolve({
    then(resolve) {
        resolve({
            then(resolve) {
                console.log(effects.join(" "));
            }
        });
    }
});

Expected Behavior

$ cat async-then.js | node
11 12 13

Actual Behavior

$ cat async-then.js | rollup --silent | node
11

$ cat async-then.js | rollup --silent
// async function
(async function () {
    return {
        get then() {
            return () => effect(11);
        }
    };
})();
const effects = [], effect = x => effects.push(x);
Promise.resolve({
    then(resolve) {
        resolve({
            then(resolve) {
                console.log(effects.join(" "));
            }
        });
    }
});

[lukastaegert]

Fix at #4120