- Fix bug in naming convention feature where using a check

- Fix bug in naming convention feature where using a check
constraint convention that includes ``constraint_name`` would
then force all :class:`.Boolean` and :class:`.Enum` types to
require names as well, as these implicitly create a
constraint, even if the ultimate target backend were one that does
not require generation of the constraint such as Postgresql.
The mechanics of naming conventions for these particular
constraints has been reorganized such that the naming
determination is done at DDL compile time, rather than at
constraint/table construction time.
fixes #3067