RFR: Traversal: Pre-evacuate code-roots and disable barriers for constants

[Roman Kennke]

We used to enable barriers on constants in Traversal GC. Reason is that
we did not pre-evacuate code-roots before coming back from
init-traversal pause, so we might have seen from-space refs. This change
implements pre-evacuating code-roots when conc-class-unloading is
enabled. It's already done when conc-class-unloading is disabled: in
this case we call process_all_roots() which also handles all code roots.

http://cr.openjdk.java.net/~rkennke/traversal-preevac-coderoots/webrev.01/

Testing: tier3_gc_shenandoah

After this change we can drop ShenandoahBarriersForConst.

Ok?