llvm seems like a nice project, but it does not fully support C++ atm (actually I think C::B parser supports more than clang does).
The problem with c-lang is that it is sponsored by big corporates like Apple and they will push it towards they directions (Objective-C/++) so dont expect to get a full CodeCompletion on silver plate
Here is the C++ status of that project:
http://clang.llvm.org/cxx_status.html - scroll down and see how in-complete it is atm
Eran