Use ErrorOS for log messages as well as error

log are also diagnostics so it seems like they should to
the same place as errors and debug messages.

Without this change when I enable --verbose those messages
go to stdout, but when I enable "-mllvm -debug" those messages
go to stderr (because dbgs() goes to stderr by default).

So I end up having to do this a lot:
 lld <args> > output_message 2>&1

Differential Revision: https://reviews.llvm.org/D41033

llvm-svn: 320427