RFR: 8310874: Runthese30m crashes with klass should be in the placeholders during verification

[Coleen Phillimore]

This is a better fix and cleanup to make placeholder use for non-parallel capable class loaders the same as the other sort of class loaders, and fixes potential problems with thread safety in the definer field of the PlaceholderEntry.   It's not easy to backport, which is why I chose the other fix for this bug.
Tested with tier1-4 with and without injected LoaderConstrantTable::verify code.

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Commit messages:
 - Removed comments and unnecessary accessors.
 - 8310874: Runthese30m crashes with klass should be in the placeholders during verification
 - 8310874: Runthese30m crashes with klass should be in the placeholders during verification

Changes: https://git.openjdk.org/jdk/pull/14889/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14889&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310874
  Stats: 59 lines in 4 files changed: 11 ins; 27 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/14889.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14889/head:pull/14889

PR: https://git.openjdk.org/jdk/pull/14889