Use -Xclang to generate .pch and completion code instead of -cc1

According to [Clang's FAQ](http://clang.llvm.org/docs/FAQ.html#i-run-clang-cc1-and-get-weird-errors-about-missing-headers), `-cc1` options are not guaranteed to be stable, and `clang -Xclang` could be a good replacement.

Furthermore, `-cc1` options can not generate completion code successfully for [cpp-netlib](http://cpp-netlib.org) header files. So, this commit is intended to fix it.