Partial index support with sqlite dialects.

Partial index support with sqlite dialects.

From https://www.sqlite.org/partialindex.html
> Partial indexes have been supported in SQLite since version 3.8.0.

Reflection does not expose the predicate of partial indexes.  The
postgresql dialect does detect such indexes and issue a warning.  I
looked into matching this level of support, but the sqlite pragma
index_info does not expose the predicate.  Getting this data would
probably require parsing the CREATE INDEX statement from sqlite_master.