CMake Improvements

[ITHelpDec]

The first change is to place .clang_complete one step above the build directory, as opposed to inside the build directory itself, with the aim of allowing clang_complete to work more seamlessly from the get-go.

The second change is to update the documentation to help users using CMake, where the current commands did not work - the missing piece of the puzzle was CMAKE_<LANG>_COMPILER_LAUNCHER.