allow-multiple-definitions should completely suppress errors instead of making them warnings.

We found that when you pass --allow-multiple-definitions or `-z muldefs`
to GNU linkers, they don't complain about duplicate symbols at all. They
don't even print out warnings on it. We emit warnings in that case.
If you pass --fatal-warnings, that difference results in a link failure.

Differential Revision: https://reviews.llvm.org/D44549

llvm-svn: 327920