Their amdgpu-install program is useless so I just created a local repository myself in /var/local and created a amdgpu.repo file in /etc/yum.repos.d/ to point to it. 

I finally figured out I needed the orca library and not the amdgpu library. I thought my RX 580 was new enough but apparently mine falls into the "legacy" OpenCL driver.

That got clinfo working for both the system version and the amdgpu version:

Platform #0: Clover
 `-- Device #0: Radeon RX 580 Series (POLARIS10, DRM 3.36.0, 5.5.13-200.fc31.x86_64, LLVM 9.0.0)
Platform #1: AMD Accelerated Parallel Processing
 `-- Device #0: Ellesmere
Platform #2: Portable Computing Language
 `-- Device #0: pthread-AMD Ryzen 5 2600 Six-Core Processor
Platform #3: Intel Gen OCL Driver

Notice the Ellesmere line. I get something similar from /opt/amdgpu-pro/bin/clinfo. 

So they both think it's working. I'm still getting clGetDeviceIDs: (-1) from FAH and I have added fahclient to both the video and render groups. Also I ensured that's actually getting passed to FAHClient which does not work when using the SysV script.

# cat /proc/$(pidof FAHClient)/status | grep Groups
Groups: 39 578 863 

39 is "video" and 578 is "render". 

For posterity here's my /etc/systemd/system/FAHClient.service:

# systemctl cat FAHClient.service
# /etc/systemd/system/FAHClient.service
# /run/systemd/generator.late/FAHClient.service

[Unit]
Description=Folding@home Client
After=remote-fs.target
After=network-online.target
Wants=network-online.target

[Service]
Type=simple
WorkingDirectory=/var/lib/fahclient
User=fahclient
ExecStart=/usr/bin/FAHClient /etc/fahclient/config.xml

Thanks,

Richard