Fix and undeprecate ReflectionType::__toString()

Fix and undeprecate ReflectionType::__toString()

Add deprecated _ZendTestClass::__toString() method to preserve
an existing test.

ReflectionType::__toString() will now return a complete
representation of the type, as it should have originally. Users
that relied on nullability being absent should have been pushed
to ReflectionNamedType::getName() by the deprecation of
ReflectionType::__toString() in PHP 7.1 / PHP 7.4.