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

[Thiago Macieira]

On Sunday, 8 July 2018 17:15:36 PDT Thiago Macieira wrote:
> This is the second time I've run into a compiler bug with that XCode's
> ancient Clang.

Third strike.

Can we PLEASE REMOVE ancient Clang that is running on macOS 10.11 in the CI 
for 5.12? Yes, I've read the email that it would leave us without 10.11 
testing. If that's the result, then do it.

And BEFORE the final downmerge?

I can't integrate changes that compile on regular Clang (Linux and FreeBSD), 
modern Apple Clang, GCC, MSVC 2015 and MSVC 2017.

If that isn't done, I can't guarantee CBOR will work. I *do not* have time to 
investigate this problem or find work arounds, so this will impact my ability 
to provide bug fixes. 

Unless the consensus is to drop the work I was doing that ran into issues. See 
https://codereview.qt-project.org/233481

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center