RFR: 8268855: Cleanup name handling in the Thread class and subclasses

[David Holmes]

Please review this small cleanup item.

We can simplify and cleanup up name() management:

- make name() return "const char *" and only cast away constness at API boundaries when essential
- add type_name() so that we can avoid code like "if (t->is_VM_Thread()) print("VMThread");
- Rename JavaThread::get_thread_name() to name() (no need for the extra indirection)

There are a couple of minor changes to the appearance of some internal threads in the hs_err log e.g.

  0x000055af03e5b1b0 WatcherThread [stack: 0x00007f685df00000,0x00007f685e000000] [id=15952]

is now:

  0x000055af03e5b1b0 WatcherThread "VM Periodic Task Thread" [stack: 0x00007f685df00000,0x00007f685e000000] [id=15952]

but this shouldn't affect anything and makes things more consistent.

Notes: 

1. While "override" is the ideal style when declaring overriding methods it has to be applied to all virtual methods in a class. So unless "override" is already used in a class, I did not start using it. I have filed a separate RFE to convert the Thread classes to use "override" consistently.
2.  While there is no need to redeclare a virtual method as "virtual" I kept to the existing style in those classes where changes were made.
3. I did not override type_name() for all the JavaThread subclasses as it seemed unnecessary, but happy to hear other views on this.

Testing (in progress):
 - All builds in tiers 1-5
 - GHA
 - tiers 1-3 as a sanity test

Thanks,
David

-------------

Commit messages:
 - Missed ShenandoahControlThread
 - 8268855: Cleanup name handling in the Thread class and subclasses

Changes: https://git.openjdk.java.net/jdk/pull/4569/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4569&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268855
  Stats: 85 lines in 16 files changed: 34 ins; 14 del; 37 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4569.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4569/head:pull/4569

PR: https://git.openjdk.java.net/jdk/pull/4569