Currently, when VIM opens a source file, YCM always defaults to
'g:global_ycm_extra_conf_file' if it exists.
This commit changes YCM's behaviour so that it first tries to find the config
file in the source file's folder (or any of its parents folder), before
falling back to 'g:global_ycm_extra_conf_file'.
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.