RFR: 8310874: Runthese30m crashes with klass should be in the placeholders during verification [v3]

[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.

Coleen Phillimore has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:

 - Merge branch 'master' into placeholder-bug
 - Don't need atomics. thanks for the explanation, David.
 - 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

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14889/files
  - new: https://git.openjdk.org/jdk/pull/14889/files/cd12e46f..9756f529

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14889&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14889&range=01-02

  Stats: 131289 lines in 2490 files changed: 48840 ins; 68343 del; 14106 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