[Xcode] Don’t use a bridging header in a framework target

[lorentey]

Oops, that was not intended to work, and in fact it produces warnings in clients.

Instead, change _sa_retain_n and _sa_release_n to Swift calling convention and call them using _silgen_name trickery when in single-module configuration.

Resolves rdar://110860922

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

[lorentey]

@swift-ci test