RFR: JDK-8293156: Dcmd VM.classloaders fails to print the full hierarchy [v3]

[Thomas Stuefe]

> Fixes a bug in the `VM.classloaders` jcmd that causes class loaders to be omitted from the output if a parent class loader never loaded any class and therefore had no associated DCmd.
> 
> The fix changes the command to not rely on the existence of a CLD structure for the loader; instead, all information (loader class name, loader name, etc) is pulled via the loader oop, which has to be always there unless its the bootstrap loader.
> 
> Also, the tests were expanded to test the display of empty loaders and empty parent loaders.
> 
> Thanks to @dholmes-ora for finding this bug.

Thomas Stuefe has updated the pull request incrementally with three additional commits since the last revision:

 - Fix test error in GHAs
 - feedback dholmes
 - childs->children

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10312/files
  - new: https://git.openjdk.org/jdk/pull/10312/files/224df5fc..3510ddcb

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10312&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10312&range=01-02

  Stats: 5 lines in 2 files changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/10312.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10312/head:pull/10312

PR: https://git.openjdk.org/jdk/pull/10312