John Marino has added the starting framework to use clang as the alternate base compiler in DragonFly. Note that it’s not hooked into the build yet. This is the first non-GCC compiler added into DragonFly, so there’s some work yet before you can have an all-clang system. This should replace GCC 4.7, which is the current alternate compiler. GCC 5.0 is the default, if you didn’t know.
Note that clang is present in dports, so it’s already been available for general use, for some time. This framework is for building DragonFly itself.