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.
Now the user has the option of writing custom logic before ycm_core.so is
loaded. This can be used to dynamically change the location of where ycm_core.so
is loaded by prepending paths to sys.path.
Very, very few people will need this feature, but I'm one of them so there.