Use PrivateLinkage now that it is safe.

Now that llvm's codegen knows to use an 'l' prefix when needed, we can just
use PrivateLinkage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201624 91177308-0d34-0410-b5e6-96231b3b80d8