Let legacy function rules adhere to @SwiftSyntaxRule

[SimplyDanny]

No description provided.

[SwiftLintBot]

	2 Warnings
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.
⚠️	This PR may need tests.

	18 Messages
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Here's an example of your CHANGELOG entry:

* Let legacy function rules adhere to `@SwiftSyntaxRule`.  
  [SimplyDanny](https://github.com/SimplyDanny)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

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