RFR: [nestmates] rename Class::isHiddenClass to Class::isHidden

[Mandy Chung]

Alex observes that a class or interface can be hidden and so he suggest 
to rename `isHiddenClass` to `isHidden` as it's not limited to class only.

`isHidden` is like `isSynthetic`, it could be a class or interface.

`isLocalClass` and `sAnonymousClass` are specific for class only because 
the Java language only has local classes and anon classes, not local 
interfaces or anon. interfaces. `isMemberClass` isn't correct since it 
can be a class or interface.

Webrev:
http://cr.openjdk.java.net/~mchung/valhalla/webrevs/hidden-classes/webrev.03-delta-rename-isHidden/

Thanks
Mandy