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

[Coleen Phillimore]

On Fri, 10 Feb 2023 19:11:17 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> 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

Thanks for reviewing Ioi!

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