Automatically install iOS Platform Support

[dfabulich]

Users who install Xcode but don't install iOS Platform Support can't pass skip checkup. (It fails on the IPA step for a missing -destination generic/platform=iOS.)

I'd been checking iOS Platform Support on autopilot while installing Xcode as part of my installation test plan #632, but users might forget to do that.

I think skip doctor could run xcodebuild -showdestinations and could even run xcodebuild -downloadPlatform iOS on the user's behalf.

[dfabulich]

Apropos skiptools/skip.dev#16