RFR: 8302108: Clean up placeholder supername code

[David Holmes]

On Thu, 9 Feb 2023 15:46:47 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.

Took me a while to get my head around the SymbolHandle mechanics for ref counting, but once I did this looks good. :) Thanks.

-------------

Marked as reviewed by dholmes (Reviewer).

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