RFR: Fix full gc with class unloading [v3]

[Roman Kennke]

On Wed, 15 Sep 2021 17:32:03 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> This pull request fixes incompatibilities between Full GC and class unloading.  In particular, when degenerated GC upgrades to Full GC, it is important to change the generation to GLOBAL.  Otherwise, certain assertions related to class unloading fail.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove irrelevant comment found during code review

Marked as reviewed by rkennke (Lead).

-------------

PR: https://git.openjdk.java.net/shenandoah/pull/67