RFR: 8273597: Rectify Thread::is_ConcurrentGC_thread() [v2]

[Per Liden]

On Mon, 13 Sep 2021 12:22:03 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Per Liden has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Fix constructor call
>>  - Add ConcurrentGCThread::cast()
>
> Marked as reviewed by stefank (Reviewer).

Thanks for reviewing @stefank and @coleenp!

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PR: https://git.openjdk.java.net/jdk/pull/5463