RFC: Adding Dockerfiles to build clang into llvm/utils

While working on providing Dockerfiles for compiling clang from source, we
found that this might also be useful to open-source the work we’ve been
doing to let anyone interested in providing a recent versions of clang in
docker images to reuse this effort.

Therefore, we want to open-source these Dockerfiles.
More info and detailed docs can be found in Phab review: https://reviews.llvm.org/D34197

For those of you, who are unfamiliar with Docker, it’s a program that uses
linux kernel features (kernel namespaces and cgroups) to create an isolated
environment on your machine, bootstrap a new root filesystem from a snapshot,
stored in a so-called docker image, and allow you to run programs in that
environment. To build this image, you define a set of instructions in a source
file, the so-called Dockerfile.
Docker is very popular and widely used to deploy binary distributions of
software to the cloud, among other things.