Convert to async function: handle type arguments to then/catch

[andrewbranch]

I think this addresses what should be a real edge case, as there’s not a great reason to supply a type argument to .then<T>(...) or .catch<T>(...), but if you do, it can change the inferred return type of the async function. Since I can’t think of a good realistic example, here’s a silly contrived one:

// logs some stuff about any argument and returns it
declare function logWhenDebugging<T>(x: T): T;

// SOURCE
// inferred return type: Promise<User>
function fetch() {
  const promiseOfAny = Promise.resolve(undefined as any);
  return promiseOfAny.then<User>(x => logWhenDebugging(x));
}

// BEFORE: we drop the type argument
// inferred return type: Promise<any>
async function fetch() {
  const promiseOfAny = Promise.resolve(undefined as any);
  const x = await promiseOfAny;
  return logWhenDebugging(x);
}

// AFTER: new variable declaration to preserve it
// inferred return type: Promise<User>
async function fetch() {
  const promiseOfAny = Promise.resolve(undefined as any);
  const x = await promiseOfAny;
  const result: User = logWhenDebugging(x);
  return result;
}

Fixes #28529