xcode: update to Xcode 9.0.1 on 10.12 and 10.13

[JCount]

• 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 tests with your changes locally?

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Xcode 9.0.1 and the 9.0.1 CLT are out for both 10.12 and 10.13.

[DomT4]

Thanks for catching this. Was on my to-do list, along with a bunch of other stuff 😅.

[JCount]

😆

[MikeMcQuaid]

👍 when our CI boxes are updated accordingly. "Fun" suggested task for another time: add a CI check to ensure docs/Xcode.md is always kept up to date (and maybe even a command to automatically spit out the output based on os/mac and os/mac/xcode

[MikeMcQuaid]

Thanks @JCount!