[foreign] RFR 8210911 jextract does not handle redundant forward, backward declarations of struct, union, enum properly

[Sundararajan Athijegannathan]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8210911
Webrev: http://cr.openjdk.java.net/~sundar/8210911/webrev.00/

PS. Piggybacking to add a TreePhase interface for visitors.

Thanks
-Sundar