RFR: 8373208: Make Atomic class template constant initializable [v2]

[Kim Barrett]

> Please review this change to Atomic to meet the requirements for constant
> initialization in cases where the value type provides the requisite support.
> This ensures that in such cases dynamic initialization won't even be
> considered a possibility when initialized with constant arguments.
> 
> Testing: mach5 tier1

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 two additional commits since the last revision:

 - Merge branch 'master' into constinit-atomic
 - constexpr Atomc ctors

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Changes:
  - all: https://git.openjdk.org/jdk/pull/28711/files
  - new: https://git.openjdk.org/jdk/pull/28711/files/e3f6f8a1..dec499b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28711&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28711&range=00-01

  Stats: 18007 lines in 308 files changed: 11474 ins; 4268 del; 2265 mod
  Patch: https://git.openjdk.org/jdk/pull/28711.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28711/head:pull/28711

PR: https://git.openjdk.org/jdk/pull/28711