RFR: Fix Zero build

[Aleksey Shipilev]

With Zero, or when C1 and C2 are disabled, current build fails with:

/pool/buildbot/slaves/sobornost/shenandoah-jdkX/build/src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp:
In constructor ‘ShenandoahBarrierSet::ShenandoahBarrierSet(ShenandoahHeap*)’:
/pool/buildbot/slaves/sobornost/shenandoah-jdkX/build/src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp:72:34:
error: ‘ShenandoahBarrierSetC1’ was not declared in this scope
              make_barrier_set_c1<ShenandoahBarrierSetC1>(),
                                  ^~~~~~~~~~~~~~~~~~~~~~
/pool/buildbot/slaves/sobornost/shenandoah-jdkX/build/src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp:72:58:
error: no matching function for call to ‘ShenandoahBarrierSet::make_barrier_set_c1()’
              make_barrier_set_c1<ShenandoahBarrierSetC1>(),

          ^
Other GCs get away with it by forward-declaring their compiler BarrierSets. We can do the same:


diff -r cd5a82e54311 src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp
--- a/src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp	Tue Jun 12 17:37:20 2018 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahBarrierSet.cpp	Tue Jun 12 18:17:43 2018 +0200
@@ -38,6 +38,9 @@
 #include "gc/shenandoah/c2/shenandoahBarrierSetC2.hpp"
 #endif

+class ShenandoahBarrierSetC1;
+class ShenandoahBarrierSetC2;
+
 ShenandoahSATBMarkQueueSet ShenandoahBarrierSet::_satb_mark_queue_set;

 template <bool UPDATE_MATRIX, bool STOREVAL_WRITE_BARRIER>


Testing: Zero x86_64 build

Thanks,
-Aleksey