New clang/llvm ports

[Jeremy Huddleston]

In r86535, I added new ports for {clang,llvm}-{2.9,3.0,3.1}.  This will allow us to support multiple versions of llvm and clang in a similar fashion to how we currently support multiple versions of gcc.  Additionally, this will allow dependent projects to rely on a *specific* version of llvm.

Once everything is flushed out, we will obsolete the old ports and have them redirect to -2.9 (for clang and llvm) or -3.1 (for clang-devel and llvm-devel).

I want to create a port select group for llvm and clang, and I'm curious what we should do.  Should we create just clang and llvm groups, or should we also create a cc group which will let someone set ${prefix}/bin/cc and ${prefix}/bin/c++?