MappedAsDataclass applies @dataclasses.dataclass unconditionally

When using the :class:`.MappedAsDataclass` superclass, all classes within
the hierarchy that are subclasses of this class will now be run through the
``@dataclasses.dataclass`` function whether or not they are actually
mapped, so that non-ORM fields declared on non-mapped classes within the
hierarchy will be used when mapped subclasses are turned into dataclasses.
This behavior applies both to intermediary classes mapped with
``__abstract__ = True`` as well as to the user-defined declarative base
itself, assuming :class:`.MappedAsDataclass` is present as a superclass for
these classes.

This allows non-mapped attributes such as ``InitVar`` declarations on
superclasses to be used, without the need to run the
``@dataclasses.dataclass`` decorator explicitly on each non-mapped class.
The new behavior is considered as correct as this is what the :pep:`681`
implementation expects when using a superclass to indicate dataclass
behavior.

Fixes: #9179
Change-Id: Ia01fa9806a27f7c1121bf7eaddf2847cf6dc5313

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