They're not available for Debian buster yet, so we have to use upstream
snapshot packages again.
In contrast to earlier, we now store the LLVM APT repository key in Git
instead of re-downloading it every time.
This requires to bump LLVM to 8 because it's the minimum supported
version by RADV.
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
