os/mac/xcode: support Xcode 11.4

[Bo98]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Fun fact: the 11.4 CLT contains the macOS 10.14 SDK despite 10.14 no longer being supported.

[Bo98]

Obviously, this'll need to wait for the CI nodes to be updated. There's several big PRs going on at the moment anyway filling up the queue.

[Bo98]

Nodes are updated now.

[MikeMcQuaid]

Thanks again @Bo98 (and for the updates 💖).