RFR: 8302108: Clean up placeholder supername code [v2]

[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.

Coleen Phillimore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Fix test for cleanup for nulling out supername when no longer used.
 - Merge branch 'master' into placeholder-cleanup
 - 8302108: Clean up placeholder supername code

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Changes: https://git.openjdk.org/jdk/pull/12495/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12495&range=01
  Stats: 53 lines in 4 files changed: 13 ins; 24 del; 16 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