Allow "final" modifier when importing a method from a trait

[nicolas-grekas]

Description

Not sure why this is not allowed currently, but it'd be nice allowing it if there aren't any blockers:

trait SomeTrait {
    function someMethod() {}
}

class SomeClass {
    use SomeTrait {
        someMethod as final;
    }
}

This currently produces: "Cannot use 'final' as method modifier"

[ndossche]

Following https://wiki.php.net/rfc/horizontalreuse (which introduced traits), this should be allowed.
The implementation was refactored in 3f8c729. That commit is the first time the "final" check appears AFAICT, but no reason was given for why. That commit seems to have landed in 5.4.11 and the NEWS for that version doesn't seem to mention something relevant to the behaviour change. I'll check what happens if we remove the check.