Add `Checker::context` method, deduplicate Unicode checks (#19609)
Summary
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This PR adds a `Checker::context` method that returns the underlying
`LintContext` to unify `Candidate::into_diagnostic` and
`Candidate::report_diagnostic` in our ambiguous Unicode character
checks. This avoids some duplication and also avoids collecting a `Vec`
of `Candidate`s only to iterate over it later.
Test Plan
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Existing tests
