I already had gcc 4.5 installed. So I just changed the search paths of clang to the ones provided by "cpp-4.5 -x c++ -v /tmp/a.cpp" - and it immediately worked.
So the problem obviously was caused by gcc 4.6.
I don't intend to use any new features of gcc 4.6 yet so in my opinion you can use your time for more important things than fixing code completion for gcc 4.6.
I don't know, what the exact differences between clang and ctags are - but as long as it works I'll stay with the current settings. Never change a running system..

Again thank you very much for helping me so fast!
Greetings,
frozen