RFR: Eliminating Shenandoah exposure to CLD

[Zhengyu Gu]

Apparently, ParallelCLDRootIterator does not need to access any 
private/protected method/field of CLD, so can eliminate CLD from diff.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/rm_cld_exposure/webrev.00/index.html


Test:
   clean fastdebug and release builds with/without precompiled headers.

Thanks,

-Zhengyu