Allow flat for join with name

Allow flat for join with name

The :paramref:`_orm.aliased.name` parameter to :func:`_orm.aliased` may now
be combined with the :paramref:`_orm.aliased.flat` parameter, producing
per-table names based on a name-prefixed naming convention.  Pull request
courtesy Eric Atkin.

Fixes: #11575
Closes: #11531
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/11531
Pull-request-sha: f85535464be7b04d5f9745848d28f87dcd248b86

Change-Id: If79679c7a9598fffe99c033894b7dffecef13939
(cherry picked from commit 0e40962bf300bb26c873d00d80813a735fb7447f)