refactor(frontend/expr): introduce ExprImpl::has_sys_func

[TennyZhuang]

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What's changed and what's your intention?

The function leave as unused.

Before my next step, I decide to do some refactors first. But the current changes of the PR seems good to merge.

Checklist

• I have written necessary rustdoc comments
• I have added necessary unit tests and integration tests
• I have added fuzzing tests or opened an issue to track them. (Optional, recommended for new SQL features Sqlsmith: Sql feature generation #7934).
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[TennyZhuang]

madsim-rs/madsim#175

[gitguardian]

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