[ty] Consider synthesized methods and `ClassVar`-qualified declarations when determining whether an abstract method has been overridden in a subclass (#23381)
## Summary
This is another standalone PR pulled out of #22898, to reduce the diff
on that PR.
Currently we emit a false-positive `abstract-method-in-final-class`
diagnostic on this class, because we do not see that the synthesized
`__lt__` method created by `@dataclass(order=True)` has overridden the
abstract `__lt__` method from the class's superclass:
```py
from abc import ABC, abstractmethod
from dataclasses import dataclass
from typing import final
class AbstractOrdered(ABC):
@abstractmethod
def __lt__(self, other): ...
@final
@dataclass(order=True)
class ConcreteOrdered(AbstractOrdered): ...
```
We also currently do not recognise `ClassVar` declarations in a subclass
as overriding an abstract method/property on a base class. While an
annotation without a binding is not sufficient at runtime as an abstract
method override, the `ClassVar` qualifier serves as an explicit
declaration from the user to the type checker that the attribute will be
accessible on the class object itself (not simply instances of the
class) at runtime. We therefore choose to treat `ClassVar` declarations
as overrides of abstract methods from superclasses.
## Test Plan
mdtests updated/extended
ruff
fb2b844c - [ty] Consider synthesized methods and `ClassVar`-qualified declarations when determining whether an abstract method has been overridden in a subclass (#23381)
