Support promise-like types in contextual return type of async function

[rbuckton]

This changes the contextual type we use for a return expression in an async function. Prior to this change, we would infer {} for the type argument to Promise below:

interface Obj { key: "value"; }

function fn1(): Promise<Obj> {
    // ok: 'Promise<T>' is contextually typed to 'Promise<Obj>'
    return new Promise(resolve => { resolve({ key: "value" }); });
}

async function fn2(): Promise<Obj> {
    // ok: return expression is contextually typed to 'Obj'.
    return { key: "value" }; 
}

async function fn3(): Promise<Obj> {
    // error: '{}' is not assignable to type 'Obj'.
    return new Promise(resolve => { 
        resolve({ key: "value" });
    });
}

This is because we use the "awaited type" Obj for the return type Promise<Obj> as the contextual type. With this change, we use Obj | PromiseLike<Obj> as the contextual type:

interface Obj { key: "value"; }

function fn1(): Promise<Obj> {
    // ok: 'Promise<T>' is contextually typed to 'Promise<Obj>'
    return new Promise(resolve => { resolve({ key: "value" }); });
}

async function fn2(): Promise<Obj> {
    // ok: return expression is contextually typed to 'Obj | PromiseLike<Obj>'.
    return { key: "value" }; 
}

async function fn3(): Promise<Obj> {
    // ok: 'Promise<T>' is contextually typed to 'Promise<Obj>'
    return new Promise(resolve => { 
        resolve({ key: "value" });
    });
}

Fixes #24629

[rbuckton]

Closing as this was inadvertently opened against 'master'.