Update file headers

[lorentey]

Update copyright dates in source file headers.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [ ] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [X] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[glessard]

Nice! So easy to forget…