Data flow: Allow for direct stores into nodes with `clearsContent`

[hvitved]

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Before this PR, store steps where the target node would clear the stored content via clearsContent would get filtered away. However, it is convenient to be able to only filter away earlier store steps, which is what this PR changes. The old semantics can be still be maintained on a per-language basis, as it is simply a matter of removing such store steps up front.

Needed in preparation for https://github.com/github/codeql/pull/15802.

[aschackmull]

Otherwise LGTM.