fix issue #22556 - shared AAs with RefCounted values don't work even after removing shared

[rainers]

allow AA.clear with shared values for backward compatibility

Long term, access to shared AAs should be deprecated, there is little safe about that. clear() in the given scenario might be an exception, as it only removes the non-shared part of the AA pointing to the shared values.

[dlang-bot]

Thanks for your pull request, @rainers!

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Testing this PR locally

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dub run digger -- build "stable + dmd#22645"

[rikkimax]

If you do a PR to deprecate shared AA's, I can bring it up at the next monthly meeting.
Ping me when you've got a solution you like.
I am noting the transitive nature of shared may be a problem here.