[lworld] RFR: 8368875: [lworld] UseParallelGC fails null narrow klass assertion failure

[Coleen Phillimore]

This seems to pass multiple stressHierarchy test invocations with -XX:+UseParallelGC, and runtime/valhalla/inlinetypes tests. I think there's probably further investigation that needs to be done but hopefully this helps with the parallel GC test failures.
Testing with tier1-4.

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Commit messages:
 - default_prototype_header isn't needed. MemAllocator::finish better have a Klass.
 - 8368875: [lworld] UseParallelGC fails null narrow klass assertion failure

Changes: https://git.openjdk.org/valhalla/pull/1655/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1655&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368875
  Stats: 41 lines in 7 files changed: 19 ins; 18 del; 4 mod
  Patch: https://git.openjdk.org/valhalla/pull/1655.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1655/head:pull/1655

PR: https://git.openjdk.org/valhalla/pull/1655