RFR: 8292671: Hotspot Style Guide should allow covariant returnts

[Roland Westrelin]

The style guide explicitly lists covariant returns as a disallowed
feature of c++. I propose we allow that feature.

This came up on the c2 side where, with JDK-8275201, I introduced some
uses of covariant returns at a number of locations in the c2 code
because I felt the code was cleaner that way. I wasn't aware it was
mentioned on the style guide. This was noticed by another openjdk
contributor and I prepared a change to revert the use of covariant
returns I had introduced:

https://github.com/openjdk/jdk/pull/9237

But it feels that change actually makes the code less clear and
comments on the PR indicates some other contributors feel the same
way.

It's unclear to me why covariant returns are disallowed in the first
place and the (few) contributors I asked don't seem to know either. So
I propose simply allowing covariant returns.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/9941/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9941&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292671
  Stats: 7 lines in 2 files changed: 3 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9941.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9941/head:pull/9941

PR: https://git.openjdk.org/jdk/pull/9941