ycm_extra_conf.py files used to import clang_helpers and then use the
PrepareClangFlags function; this is now unnecessary since the logic from that
function has been moved to flags.py. The old PrepareClangFlags function is still
there (it just returns the flags it gets) for the sake of backwards
compatibility with old ycm_extra_conf.py files.
We now use the compilation working directory for a file that is specified in the
CompilationDatabase. We don't actually change the working directory of the
process, even temporarily (that would be annoying to users); we munge the flags
coming from the database so that all the relative paths in them are resolved to
absolute ones.
