RFR: 8282668: HotSpot Style Guide should permit unrestricted unions

[Kim Barrett]

Please review this change to permit the use of "unrestricted unions"
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2544.pdf) in HotSpot
code.

This permits any non-reference type to be used as a union data member, as well
as permitting static data members in named unions. There are various classes
in HotSpot that might be able to take advantage of this new feature.

An example is the aarch64-specific Address class. It presently contains a
collection of data members. For any given instance, only some of these data
members are initialized and used. The `_mode` member indicates which. So it's
effectively a kind of discriminated union with the data unpacked and not
overlapping, with `_mode` being the discrimenant. A consequence of the current
implementation is that some compilers may generate warnings under some
circumstances because of uninitialized data members. (I ran into this problem
with gcc when making an otherwise unrelated change to one of the member
types.) This Address class could be made smaller (so cheaper to copy, which
happens often as Address objects are frequently passed by value) and usage
made clearer, by making it an actual union. But that isn't possible with the
C++03 restrictions.

Another example is the RelocationHolder class, which is effectively a union
over the various concrete Relocation types, but implemented in a way that
has some issues (JDK-8160404).

Testing:
I've tried some examples without running into any problems.  This included
some experiments with RelocationHolder for JDK-8160404.

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 Friday 18-Mar-2022 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.

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Commit messages:
 - update html
 - unrestricted unions

Changes: https://git.openjdk.java.net/jdk/pull/7704/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7704&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282668
  Stats: 4 lines in 2 files changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7704.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7704/head:pull/7704

PR: https://git.openjdk.java.net/jdk/pull/7704