RFR: 8278791: Rename ClassLoaderData::holder_phantom

[Coleen Phillimore]

Trivial change to change the name of holder_phantom.
Tested with cds/appcds tests locally.

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Commit messages:
 - Fix comment above CLD::holder().
 - 8278791: Rename ClassLoaderData::holder_phantom

Changes: https://git.openjdk.java.net/jdk/pull/6834/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6834&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278791
  Stats: 11 lines in 5 files changed: 1 ins; 3 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6834.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6834/head:pull/6834

PR: https://git.openjdk.java.net/jdk/pull/6834