Use coercion rules for aliased() against select/union constructs

Use coercion rules for aliased() against select/union constructs

Calling :func:`_orm.aliased` against a :func:`_sql.select` or
:func:`_sql.union` / :class:`_sql.CompoundSelect` construct, which
previously failed with an obscure ``AttributeError`` regarding a missing
``.mapper`` attribute, now raises when using SQLAlchemy 2.1, and emits a
deprecation warning under SQLAlchemy 2.0 as it coerces the construct into a
subquery instead.  This matches the behavior of other similar implicit
SELECT-to-FROM coercions.  Pull request courtesy Rens Groothuijsen.

Fixes: #6274
Closes: #12433
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12433
Pull-request-sha: 416dde8509ac05b209400603d9e00bc82aa47c79

Change-Id: I63bdec71074b81fb85bf29e6ca0dd81cbe3f8cb3
(cherry picked from commit 16177b8c73e492e5adaad8f51095f9981831da41)