RFR: 8293315: Add back logging for Placeholders

[Coleen Phillimore]

I lost some information in the logging code for PlaceholderTable that I really want when I converted it to use a ResourceHashtable, so this minor patch adds it back.
Tested locally with -Xlog:class+load+placeholders=trace, tier1 in progress.

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Commit messages:
 - 8293315: Add back logging for Placeholders

Changes: https://git.openjdk.org/jdk/pull/10145/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10145&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293315
  Stats: 9 lines in 2 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/10145.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10145/head:pull/10145

PR: https://git.openjdk.org/jdk/pull/10145