RFR: 8252584: HotSpot Style Guide should permit alignas [v2]

[Julian Waters]

> Add alignas to the permitted features set with some restrictions. (Thanks @kimbarrett for the help)

Julian Waters 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 'openjdk:master' into alignas
 - HotSpot Style Guide changes

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11404/files
  - new: https://git.openjdk.org/jdk/pull/11404/files/cec044b4..43683f1b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11404&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11404&range=00-01

  Stats: 5941 lines in 250 files changed: 3584 ins; 1175 del; 1182 mod
  Patch: https://git.openjdk.org/jdk/pull/11404.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11404/head:pull/11404

PR: https://git.openjdk.org/jdk/pull/11404