RFR: JDK-8293156: Dcmd VM.classloaders fails to print the full hierarchy [v2]

[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 one additional commit since the last revision:

  cjplummer feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10312/files
  - new: https://git.openjdk.org/jdk/pull/10312/files/0f5de199..224df5fc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10312&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10312&range=00-01

  Stats: 20 lines in 1 file changed: 10 ins; 0 del; 10 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