Add support for pseudo-abstract/self-referencing constants

[El-Virus]

PHP doesn't support abstract constants, so using self-referencing constants in traits in order to force implementer classes to define the constant is not unheard of.
An example of such an implementation would be:

trait Greeter {
    private const string WHO = self::WHO;

    public static function greeting() : string {
        return "Hello, " . static::WHO;
    }
}

class JackGreeter {
    use Greeter;
    private const string WHO = 'Jake';
}

Unfortunately PHPStan does not currently take into consideration that self-referencing constants are (usually) meant to be overridden.

This Pull Request omits the Constant Type check for self-referencing constants.

[staabm]

interessting. I have never seen this in practice before.

[ondrejmirtes]

I've seen this before in the issue tracker but can't find it now. However, this is an awful hack and I don't plan to support it. Your code is broken. Use abstract method instead.