[Development] Upgrade the XCode running in the CI's macOS 10.11

[Thiago Macieira]

This is the second time I've run into a compiler bug with that XCode's ancient 
Clang.

Can we please upgrade the XCode installation there to the latest available for 
macOS 10.11? 

If that's the latest, can we declare that building Qt on 10.11 is no longer 
supported for Qt 5.12? I'm not asking about deployment, only building.
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center