RFR: 8252582: HotSpot Style Guide should permit variable templates

[Kim Barrett]

Please review this change to the HotSpot Style Guide to permit the use of
variable templates. This is a C++14 feature that, while fairly simple, didn't
get into the initial permitted for HotSpot list because I just didn't get to
it and nobody asked for it. And it has languished on the "undecided" list for
the same reasons. But recently I've had reason to want to use this feature a
couple of times, so I'm now proposing we allow it.

I merged it into the section for Inline Variables (a C++17 feature), as the
two are closely related. Variable templates were even listed as one of the
workarounds being used to make up for the lack of inline variables.

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Commit messages:
 - permit variable templates

Changes: https://git.openjdk.org/jdk/pull/27240/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8252582
  Stats: 50 lines in 2 files changed: 15 ins; 7 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/27240.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27240/head:pull/27240

PR: https://git.openjdk.org/jdk/pull/27240