RFE(XS): Purge previous class definitions

[Zhengyu Gu]

I don't see issue with class redefinition in Shenandoah, so let's purge 
early class definitions during class unloading.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/purge_prev_class_def/webrev.00/

Test:

   fastdebug and release:

   jtreg runtime/RedefineObject and runtime/RedefineTests

Thanks,

-Zhengyu