[foreign] 8219821: Extracted enum constants should be grouped into an interface

[Sundararajan Athijegannathan]

Please review.

When _h "static forwarder" class is generated, each (named) enum is 
mapped to an inner interface of the forwarder class.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219821
Webrev: https://cr.openjdk.java.net/~sundar/8219821/webrev.00/

Thanks,
-Sundar