Fix missing standard include files problem

This fixes the problem that completion doesn't work because of missing
standard include files.

I solved this problem by using the `clang` *driver* instead of the `clang -cc1` *frontend*.
Clang's [FAQ](http://llvm.org/releases/3.4/tools/clang/docs/FAQ.html#id2) says it's not good idea for a user to use the frontend directly.