RFR: Fix full gc with class unloading [v3]

[Kelvin Nilsen]

> 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

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

Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/67/files
  - new: https://git.openjdk.java.net/shenandoah/pull/67/files/82e1e6e0..513963b3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=67&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=67&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/67.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/67/head:pull/67

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