RFR: JDK-8222259: Shenandoah: Pre-evacuate string-dedup roots in Traversal GC

[Roman Kennke]

We need to pre-evacuate string-dedup roots in Traversal GC, just like we
do in conc-mark-GC, otherwise we might get from-space objects there, and
hit asserts later:

# Internal Error
(/home/jenkins/workspace/nightly/jdk-jdk/src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp:168),
pid=78715, tid=78721
# Error: Shenandoah assert_not_in_cset_loc failed; Interior location
should not be in collection set

More details in the bug:
https://bugs.openjdk.java.net/browse/JDK-8222259

Fix:
http://cr.openjdk.java.net/~rkennke/JDK-8222259/webrev.00/

Testing: hotspot_gc_shenandoah

I'm doing this somewhat blindly because I couldn't reproduce it locally,
but I'm fairly certain that this is the fix.