Fix #3053: renamed toString to __toStringHelper in the reflection extension

I tried to fix #3053 by renaming `toString()` (which caused conflicts with user libs) to `__toStringHelper()` (which hopefully should create much less conflicts.

First time contributing, so I'm hoping that I did everything correctly :)
I believe it is, I haven't changed this comment. __toStringHelper is indeed the implementation used for __toString methods.

> Le 30 juil. 2014 à 17:31, Markus Staab <notifications@github.com> a écrit :
> 
> In hphp/runtime/ext/reflection/ext_reflection_hni.php:
> 
> > @@ -340,7 +340,7 @@ public function getClosureScopeClass(): ?ReflectionClass {
> >    }
> >  
> >    // Implementation of __toString
> this comment is not correct
> 
> —
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