Fix ReflectionEnum::newInstance() return type

[ondrejmirtes]

PHP does not complain about this (because parent has object return type) but it's wrong. I found out about it after introducing generics to ReflectionEnum: phpstan/phpstan-src@9ce8faf

 ------ ------------------------------------------------------------------------- 
  Line   src/Reflection/Adapter/ReflectionEnum.php                                
 ------ ------------------------------------------------------------------------- 
  364    Return type                                                              
         (Roave\BetterReflection\Reflection\Adapter\ReflectionEnum) of method     
         Roave\BetterReflection\Reflection\Adapter\ReflectionEnum::newInstance()  
         should be compatible with return type (UnitEnum) of method               
         ReflectionClass<UnitEnum>::newInstance()                                 
 ------ ------------------------------------------------------------------------- 

[Ocramius]

Wait, did this pass before because self is a subtype of object? Sounds like we never call this API in our tests? 🤔

[ondrejmirtes]

@Ocramius It's fine even when called because the method always throws...

[Ocramius]

Waiting for CI, then shipping this 👍

[Ocramius]

Thanks @ondrejmirtes!