oh dear, this is a proper mess isn't it? I'm sorry it's not working for you. My Xubuntu is on Mesa 19.1.0-devel, LLVM 7.0.1 and I'm getting the assert as well. Annoyingly my Radeon R9 380 isn't fully supported by Gallium3D and I only get a few limited resolutions which is making this hard to debug (I usually use the amd-gpu proprietary drivers). I'll do some more digging today and maybe get in contact with the BGFX developers to see if they have an hints on a work around.
Thanks for your patience with this. Do you happen to remember which version of Mesa etc. you used when it was working?