Don't hard-evaluate non-ORM @declared_attr for AbstractConcreteBase

Fixed bug where using :class:`.declared_attr` on an
:class:`.AbstractConcreteBase` where a particular return value were some
non-mapped symbol, including ``None``, would cause the attribute
to hard-evaluate just once and store the value to the object
dictionary, not allowing it to invoke for subclasses.   This behavior
is normal when :class:`.declared_attr` is on a mapped class, and
does not occur on a mixin or abstract class.  Since
:class:`.AbstractConcreteBase` is both "abstract" and actually
"mapped", a special exception case is made here so that the
"abstract" behavior takes precedence for :class:`.declared_attr`.

Change-Id: I6160ebb3a52c441d6a4b663c8c9bbac6d37fa417
Fixes: #3848

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