RFR: 8302108: Clean up placeholder supername code

[Coleen Phillimore]

Please review change to make PlaceholderEntry::_supername a SymbolHandle to correctly refcount the symbol rather than have ad-hoc code.  It also moves the private functions to private and asserts invariant that find_and_remove must always find a matching entry.  Finally, this also more eagerly removes the supername symbol from the placeholder entry, so I have to fix the test in https://github.com/openjdk/jdk/pull/12491 before pushing this.
Tested with tier1-4.

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Commit messages:
 - 8302108: Clean up placeholder supername code

Changes: https://git.openjdk.org/jdk/pull/12495/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12495&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302108
  Stats: 51 lines in 3 files changed: 15 ins; 22 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/12495.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12495/head:pull/12495

PR: https://git.openjdk.org/jdk/pull/12495