RFR: 8302189: Mark assertion failures noreturn [v3]

[Kim Barrett]

> Also 8302799: Refactor Debugging variable usage for noreturn crash reporting

Kim Barrett 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 four additional commits since the last revision:

 - Merge branch 'master' into noreturn2
 - make Debugging::_enabled a nesting counter
 - new implementation of Debugging
 - noreturn attributes

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12845/files
  - new: https://git.openjdk.org/jdk/pull/12845/files/f296ab62..59295614

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12845&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12845&range=01-02

  Stats: 18174 lines in 538 files changed: 7268 ins; 8749 del; 2157 mod
  Patch: https://git.openjdk.org/jdk/pull/12845.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12845/head:pull/12845

PR: https://git.openjdk.org/jdk/pull/12845