Link runtime compatibility libs on darwin if needed

[owenv]

This also fixes Interpreter/objc_class_properties_runtime from the integration tests

[owenv]

@swift-ci test

[DougGregor]

Thank you for doing this; this looks good as a direct port of the logic from the C++ driver. It does make me wonder if we should get this information from the frontend directly. For example, -print-target-info given the target triple will tell us runtime compatibility version, resource directory, etc., and can certainly be extended. For example:

  "target": {
    "triple": "arm64-apple-ios13.0",
    "unversionedTriple": "arm64-apple-ios",
    "moduleTriple": "arm64-apple-ios",
    "swiftRuntimeCompatibilityVersion": "5.1",
    "librariesRequireRPath": false
  },

[owenv]

@DougGregor that's a good idea I hadn't thought of, it would be nice to avoid duplication between the driver & frontend. I'll look into integrating -print-target-info in a follow-up, I think it'll be easier now that there are a few unit tests to verify against.