RFR (trivial) 8213211: [BACKOUT] Allow Klass::_subklass and _next_sibling to have unloaded classes

[jesper.wilhelmsson at oracle.com]

Looks good!
/Jesper

> On 31 Oct 2018, at 20:16, coleen.phillimore at oracle.com wrote:
> 
> hg backout succeeded without any problems.   I made ErikO the reviewer in the hg commit.
> 
> open webrev at http://cr.openjdk.java.net/~coleenp/8213211.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8213211
> 
> There are several hangs.  One identified cause but others are iffy. Would rather backout and go on vacation and fix it when I'm back and mach5 is stable.
> 
> Thanks,
> Coleen