array_filter with a first-class callable can narrow its return type

[takaram]

Closes phpstan/phpstan#11337

[staabm]

Just a random idea maybe worth investigating.

would it be possible to rewrite a first-class-callable at the AST level to e.g. a regular callable (or closure::fromCallable), so we don't need special cases für callable and first-class-callables in every phpstan extension…?

otherwise we would need a similar fix we have in this PR in other extensions, e.g. in array_map for phpstan/phpstan#9224

[ondrejmirtes]

@staabm You're on the right track. I've recently started adding these things to CallableParametersAcceptor. If we added the right method to that interface, like public function getAsserts(): Assertions, we could simplify the extension by a lot + it'd help other places to be analysed correctly as well.

These assertions would be propagated to ClosureType when creating it from function or method in MutatingScope:.createFirstClassCallable.

This PR fixes array_filter extension, but a proper long-term solution is what I described here.