Further reflection api improvements

[Christian Sciberras (uuf6429)]

Changes:

• fixes a mistake in my previous reflection PR regarding getAttributes() return type
• changes method name strings to non-falsy-string to align with phpstan stubs
• improves type hinting of reflaction class initializers (heavily inspired from phpstan stubs)
• since reflection classes are now mainly generic, fixed the ones with undefined templates
• a few other minor fixes

Some relevant links:

• https://phpstan.org/r/78cf39ee-1d76-4e14-b2a6-990d856f994c
• https://3v4l.org/S0Oon#v8.5.7

[Christian Sciberras (uuf6429)]

Ondřej Mirtes (@ondrejmirtes) when you have time, could you maybe let me know if I'm missing something? I'd gladly do the relevant changes on phpstan side as well (as I started already).

My remaining question so far is the 2x stub definitions in phpstan regarding the *WithLazyObjects cases, if/how I should handle that (in particular, I don't understand why they would require @template-covariant).