llvm6: Update x86_64 FreeBSD image to Ubuntu 18.04

Looks like the clang with 16.04 fails to compile LLVM 6, but it looks like clang
in 18.04 can indeed compile LLVM 6. This fix doesn't apply for the i686 version
of FreeBSD which I'm still looking into.

src/ci/docker/dist-x86_64-freebsd/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:16.04
+FROM ubuntu:18.04
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
   clang \