RFR: Include shenandoahSupport.hpp instead of shenandoahBarrierSetC2.hpp in C2's classes.cpp

[Aleksey Shipilev]

On 12/2/18 1:34 PM, Roman Kennke wrote:
> Following up Vladimir's review, we can reduce include in classes.cpp to
> just shenandoahSupport.hpp to get the various ShenandoahXYZNode's
> declarations.
> 
> Testing: hotspot_gc_shenandoah
> 
> OK?

We actually deliberately include only barrierSetC2 headers, treating our shenandoahSupport,
brooksPointer, etc. a mere implementation detail. I think this is the same reason why ZGC includes
zBarrierSetC2, and we want to be symmetrical there?

-Aleksey