PerformPromiseThen should unconditionally set [[PromiseIsHandled]] to true

[tschneidereit]

Right now, this only happens if the promise is already rejected. That leads to Promises never being marked as handled in scenarios like the following:

let reject;
const p = new Promise((res, rej)=>{reject = rej});
p2 = p.then(f);
reject();

/cc @domenic

[tschneidereit]

Dupe of #383. That went well.

[tschneidereit]

Dupe of #383. That went well.