Proposed patch for review

[Jonathan Gibbons]

I've posted a patch from our newest contributor, Vicente Zaldivar.
The patch is a cleanup to the JCTree.tag values, to use enums
instead of small integers.

You can see the patch here:
http://cr.openjdk.java.net/~jjg/6921494/webrev.00/

Thanks to Vicente for the contribution. Review feedback here or
to Vicente, please.

-- Jon