Add an Xcode project exercising native builtins in a monomodule configuration

[lorentey]

This is going to ship as is, with no promises about it remaining in future versions. It is going to be a load bearing component of the package, though — just not necessary a stable one.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] 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