Incorrect hover on anon fn named args

[bmewburn]

Although unknown named argument was fixed, hovering over "files" still says "@var unset $files" and also the type is assumed for the closure from the default value @var \Closure(array $files, string $css_dir = 'scss'): void $loader, the css_dir argument does not define "string" as type, it defines no type, so it should be "mixed", only the default value is string.

Originally posted by @realjjaveweb in #2801 (comment)