Rename internal passRetained helper to _passRetained

[xedin]

This avoids ambiguity with a public version of the same API that is part of a Swift standard library and returns a non-optional result.

Checklist

• I've read the Contribution Guidelines
• 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).
• I've verified that my change does not break any existing tests.
• I've updated the documentation if necessary.

Resolves: rdar://180441772

[lorentey]

Thanks! 👍