RFR: 8074101: Add verification that all tasks are actually claimed during roots processing [v2]

[Albert Mingkun Yang]

> The first commit removes some obsolete enum items, while the second commit adds the verification logic. Commit 2 introduces some "empty" task claims for the verification logic, explicitly marked in the comments.
> 
> Test: hotspot_gc

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2046/files
  - new: https://git.openjdk.java.net/jdk/pull/2046/files/4fc22b48..938dce1d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2046&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2046&range=00-01

  Stats: 71 lines in 3 files changed: 35 ins; 25 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2046.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2046/head:pull/2046

PR: https://git.openjdk.java.net/jdk/pull/2046