[swift5] Add access control to the properties of validation rules

[maoyama]

Related #14218
I added access control to make the validation rules of the generated model accessible from external modules.

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• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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[4brunu]

Looks good to me 👍