RFR (S) 8213107: Make ClassLoaderDataGraph iterator skip unloaded CLDs

[coleen.phillimore at oracle.com]

Summary: with concurrent class unloading, the CLDG could contain 
unloaded CLDs while iterating in a safepoint

This fixes the parallel class loading tests (which unfortunately aren't 
in the open tests) with concurrent class unloading. Redefinition was 
iterating through the CLDG and barfing on unloaded classes.

Tested with pcl tests, test/hotspot/jtreg/runtime, jfr tests, and 
test/hotspot/jtreg/vmTestbase/nsk which includes jvmti and jdi tests.   
These things iterate through CLDG.

open webrev at http://cr.openjdk.java.net/~coleenp/8213107.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8213107

Thanks,
Coleen