I just wanted to put this at the top of the boards in case anyone missed it in a buried thread.
As reported by others the following fixed my loading screen error (using an AMD R9 390)
-DO AT YOUR OWN RISK-
Windows 10:
Go to: C:\Program Files (x86)\Steam\steamapps\common\MGS Test App 6\data\hardware. Find the text file for your graphics card
AMD Users: GPU_AMD.txt.
INTEL Users: GPU_Intel.txt
NVIDIA Users: GPU_Nvidia.txt
Rename the file for your graphics card.
I renamed mine GPU_AMD_TEMP.txt. so that it would be easier to revert back to the old name if needed.
Relaunch Halo. Wait (because it does take awhile), and eventually it loads.
Hopefully this helps at least one other!