so that we don't have "fprintf (stderr, ...)" calls sprinkled everywhere.
Changed all needed locations over to using this.

For non-darwin, we log to stderr only. On darwin, we log to stderr _and_
to ASL (Apple System Log facility). This will allow GUI apps to have a place
for these error and warning messages to go, and also allows the command line
apps to log directly to the terminal.

llvm-svn: 147596