Switch llvm build to Ubuntu and use clang+libc++
This MR switches the LLVM build to use an Ubuntu based image which has clang7 + libc++ in it. This will fix the build error in !520 (merged) due to a mismatch between clang and GNU's stdlib.
I was unable to build ROOT with clang+libc++, so I had to disable the TGeo and DD4hep plugins from this build. Even if ROOT builds, I don't think DD4hep is compatible with libc++, since as far as I see it, they code in macros specific to GNU's ABI...