Pros:
* Less fragile due to not mixing packages from stretch and buster
* No longer need to use third-party LLVM packages
* The buster image now uses GCC 8 for C++ as well (previously 6 for C++,
8 for C), allowing to drop some hacks
Con:
* The stretch image now only uses GCC 6 for C as well as C++
* Need separate jobs for testing old LLVM versions
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Michel Dänzer