Base clang is already version 3.1. Why are you going through all that trouble of ports-this and base-that, just for the minute difference of 3.1 vs 3.1-2?
Use clang provided in base, delete the port install (lang/clang*) and be done with it. You can even disable the build for gcc42 in buildworld since you don't plan to use it (come ports will require gcc, so you will have to install gcc46 or such from ports).
If you want to be adventurous and cutting edge that badly, switch from RELENG to STABLE - although it's not HEAD, it's still a development branch and new stuff gets placed in there fairly quickly.
# svn co [url=svn://svn.freebsd.org/base/stable/9]svn://svn.freebsd.org/base/stable/9[/url] /usr/src