Add __clause_element__ to ColumnProperty

Add __clause_element__ to ColumnProperty

Added a ``__clause_element__()`` method to :class:`.ColumnProperty` which
can allow the usage of a not-fully-declared column or deferred attribute in
a declarative mapped class slightly more friendly when it's used in a
constraint or other column-oriented scenario within the class declaration,
though this still can't work in open-ended expressions; prefer to call the
:attr:`.ColumnProperty.expression` attribute if receiving ``TypeError``.

Fixes: #4372
Change-Id: I5d3d1adb9c77de0566298bc2c46e9001d314b0c7