[swift] omit unknown enum from collections

Summary:
Following up on comments from D1618817. This will omit the unknown values from
collections

Test Plan: mvn clean install

Reviewers: alandau, pickett, haijunz, andrewcox, paramr

Reviewed By: andrewcox

Subscribers: andrewcox

Differential Revision: https://phabricator.fb.com/D1626427