Use error() instead of warn() to report undefined symbols.

I believe the reason why we used warn() instead of error() to report
undefined symbols is because the older implementation of error() exitted
immediately. Here, we want to find as many undefined symbols as we can,
so I chose to use warn() instead of error().

Now error() does not exit immediately, so it doesn't make sense to keep
them as warnings.

Differential Revision: https://reviews.llvm.org/D38652

llvm-svn: 315131