Clang + VS6

[JohnsterID]

Add clang workflow to CI similar to https://github.com/LoneGazebo/Community-Patch-DLL/blob/master/build_vp_clang.py

Vox Populi have to use v90 (Visual Studio 2008 SP1) libs and includes to link with Civ 5. The project has been able to move towards using clang whilst using v90 so potentially GeneralsGameCode could use clang with vs6.