RFR (S) 8218755: [REDO] Symbol leak in prepend_host_package_name

[coleen.phillimore at oracle.com]

Summary: fix Symbol refcounting again, add comment and a test.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8218755.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8218755

I hope this is better.  Ran hs-tier1-6 and a new test!

Thanks,
Coleen