[libc++] Rework buildbot configuration for the greater good.

This commit rewrites/removes the docker files used to create
the libc++ buildbots.

The major changes in this patch are:

1. Delete Dockerfiles used to build compilers. These have moved to
   github.com/efcs/compiler-images

2. Minimize the llvm-buildbot docker image. Instead of running the
buildbots from a committed docker image, the builders now build the
image on startup. This means changes to the docker file automatically
propogate to the builders (within ~24 hours without restart).

3. Version the compilers used by the builders. This means the bots
won't start failing because the apt.llvm.org clang package updated.