ubuntu 16.04: Update clang and llvm libraries

libclang 3.9 or greater is necessary to build Rust C extensions that use bindgen. Because Ubuntu 16.04 is still a valid build target until omnibus-gitlab#6211 (closed) is closed, install the more recent version so that Rust extensions can build.

This came out of a test build: https://dev.gitlab.org/gitlab/omnibus-gitlab/-/jobs/15620261