Avoid false error on is_subclass_of

[VincentLanglet]

Closes phpstan/phpstan#13713

Solves phpstan/phpstan#6305 (comment)

[phpstan-bot]

This pull request has been marked as ready for review.

[VincentLanglet]

This issue was more complicated than expected... I'm not against your opinion/review @staabm :)

[VincentLanglet]

I dunno what to do about the webmozart-assert failure because I don't report anymore

Call to static method Webmozart\Assert\Assert::implementsInterface() with class-string<WebmozartAssertImpossibleCheck\Bar>|WebmozartAssertImpossibleCheck\Bar and 'WebmozartAssertImpossibleCheck\\Bar' will always evaluate to true.

Technically, because a implementsInterface check is called before it's indeed always true.
But strictly looking at the type, calling

class-string<WebmozartAssertImpossibleCheck\Bar>|WebmozartAssertImpossibleCheck\Bar

And currently, phpstan/phpstan-webmozart-assert#194 is showing the issue