[swift] return null if unknown enum value is passed

Summary:
Allows unknown enum values to not cause exceptions in parsing.
Fixes reflection codec for required values.

Test Plan: mvn clean install

Reviewers: alandau, haijunz, andrewcox

Reviewed By: andrewcox

Subscribers: paramr, pickett, andrewcox

Differential Revision: https://phabricator.fb.com/D1618817