allow CTE to be direct DML target

allow CTE to be direct DML target

Implemented support for a :class:`_sql.CTE` construct to be used directly
as the target of a :func:`_sql.delete` construct, i.e. "WITH ... AS cte
DELETE FROM cte". This appears to be a useful feature of SQL Server.

The CTE is now generally usable as a DML target table however
it's not clear if this syntax is valid beyond the use case of
DELETE itself.

Fixes: #6464
Change-Id: I3aac6fae2a1abb39bc0ffa87a044f5eb4f90f026