RFR: Upstream merge to jdk-12+23

[Aleksey Shipilev]

On 12/6/18 1:44 PM, Roman Kennke wrote:
> Let's move to new tag jdk-12+23. Merge was trivial.
> 
> Notable:
> - 8214476: ZGC: Build ZGC by default
> 
> Testing: hotspot_gc_shenandoah
> 
> http://cr.openjdk.java.net/~rkennke/upstream-jdk12-merge-2018-12-06/outgoing.txt

Looks good.

-Aleksey