RFR: 8254050: HotSpot Style Guide should permit using the "override" virtual specifier

[Kim Barrett]

Please review and vote on this change to the HotSpot Style Guide to permit
the use of `override` virtual specifiers.  The virtual specifiers `override`
and `final` were added in C++11, and use of `final` is already permitted in
HotSpot code.

Using the `override` specifier provides error checking that the function is
indeed overriding a virtual function declared in a base class.  This can
prevent some often surprisingly difficult to spot bugs.

This is a modification of the Style Guide, so rough consensus among
the HotSpot Group members is required to make this change.  Only Group
members should vote for approval (via the github PR), though reasoned
objections or comments from anyone will be considered.  A decision on
this proposal will not be made before Tuesday 30-Mar-2021 at 12h00 UTC.

Since we're piggybacking on github PRs here, please use the PR review
process to approve (click on Review Changes > Approve), rather than
sending a "vote: yes" email reply that would be normal for a CFV.
Other responses can still use email of course.

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Commit messages:
 - permit override specifier

Changes: https://git.openjdk.java.net/jdk/pull/3021/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3021&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254050
  Stats: 8 lines in 1 file changed: 5 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3021.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3021/head:pull/3021

PR: https://git.openjdk.java.net/jdk/pull/3021