AMD has released a new Radeon RX 5700 Series driver for Ubuntu Linux 18.04 LTS

Radeon™ Software for Linux® 19.30 for Radeon RX 5700 Series Highlights
Provides support for the Radeon RX 5700 Series of graphics cards

Known Issues
[AMDGPU-Pro] Dota 2 players might experience instability running the game under Vulkan
[AMDGPU-All Open] Under certain conditions UVD might not engage during 4K HEVC media playback
[AMDGPU-All Open] Corruption observed when resizing MPV Media player
[AMDGPU-All Open] Potential instability in some older OpenGL titles

  AMD Radeon Software for Linux 19.30 for Radeon RX 5700 Series