[6.x] Improved the reflector

[GrahamCampbell]

1. Fixed treatment of union types in PHP 8. Foo|null and ?Foo will be treated as "Foo", Foo|Bar will be treated as null.
2. Fixed treatment of self typing. self declared in a parameter in a method defined in class Foo will be treated as "Foo", even if the reflection parameter was generated from querying a class Bar that extends or implements Foo. This mimics the behaviour of getClass on PHP 7, which was removed in PHP 8 (which is what we are here to simulate).

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Closes #33177.