[RFC] Ensure correct magic methods' signatures when typed

Hello internals.

I'd like to discuss with you about https://github.com/php/php-src/pull/4177.

I'll write a formal RFC for this during the week.

Open question:

public function __constructor() {}

vs.

public function __constructor(): void {}

The same goes for __destruct, __clone and others that do not have
return type nowadays.