Don't discard leftovers from surface_selectables

Don't discard leftovers from surface_selectables

Fixed regression introduced in 1.3.2 for the PostgreSQL dialect, also
copied out to the MySQL dialect's feature in 1.3.18, where usage of a non
:class:`_schema.Table` construct such as :func:`_sql.text` as the argument
to :paramref:`_sql.Select.with_for_update.of` would fail to be accommodated
correctly within the PostgreSQL or MySQL compilers.

Fixes: #5729
Change-Id: I265bcc171f0eb865ac3910ee805b162f3b70e2c1
(cherry picked from commit 6125dfff91ca0093a00d78804917240617825c44)