RFR: 8273597: Rectify Thread::is_ConcurrentGC_thread()

[Per Liden]

`Thread::is_ConcurrentGC_thread()` behaves differently to all other `Thread::is_xxx_thread()` functions, in the sense that it doesn't directly map to a distinct `Thread` sub-class. Instead, `is_ConcurrentGC_thread()` can today return true for both `ConcurrentGCThread` and `GangWorker`. These two classes have no super/sub-class relation. This is confusing and and potentially dangerous.

It would be reasonable to think that code like this would be correct:


  if (thread->is_ConcurrentGC_thread()) {
    conc_thread = static_cast<ConcurrentGCThread*>(thread);
    ...
  }


but it's not, since we might try to cast a `GangWorker` to a `ConcurrentGCThread`. And again, these two classes have no super/sub-class relation.

I propose that we clean this up, so that `is_ConcurrentGCThread()` only returns true for threads inheriting from `ConcurrentGCThread`. The main side-effect is that a handful of asserts need to be adjusted. In return, the code example above would become legal, and we can also remove some cruft from `WorkGang`/`GangWorker`.

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Commit messages:
 - 8273597: Rectify Thread::is_ConcurrentGC_thread()

Changes: https://git.openjdk.java.net/jdk/pull/5463/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5463&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273597
  Stats: 53 lines in 17 files changed: 6 ins; 26 del; 21 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5463.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5463/head:pull/5463

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