organize datetime types for correct impls

organize datetime types for correct impls

Fixed bug in SQL Server ``DATETIMEOFFSET`` where the ODBC implementation
would not generate the correct DDL, for cases where the type were converted
using the ``dialect.type_descriptor()`` method, the usage of which is
illustrated in some documented examples for :class:`.TypeDecorator`, though
not necessary for most datatypes. Regression was introduced by
:ticket:`6366`. As part of this change, the full list of SQL Server date
types have been amended to return a "dialect impl" that generates the same
DDL name as the supertype.

Fixes: #7129
Change-Id: I7d9bea54c0c38e16d1a6ad978cca996006a1b624