gitlab-ci: Test against LLVM / clang 9 on x86
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 instead of re-downloading it every time.
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 instead of re-downloading it every time.