Reflect mssql/postgresql filtered/partial indexes

Reflect mssql/postgresql filtered/partial indexes

Added support for inspection / reflection of partial indexes / filtered
indexes, i.e. those which use the ``mssql_where`` or ``postgresql_where``
parameters, with :class:`_schema.Index`.   The entry is both part of the
dictionary returned by :meth:`.Inspector.get_indexes` as well as part of a
reflected :class:`_schema.Index` construct that was reflected.  Pull
request courtesy Ramon Williams.

**Have a nice day!**
Fixes: #4966
Closes: #5504
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5504
Pull-request-sha: b3018bac987081193b2e65cfdb6aeb7d5d270fcd

Change-Id: Icbb2f93d1545700718ccb5222097185b815f5dbc