[PECOFF] Implement /alternatename weak symbols.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196754 91177308-0d34-0410-b5e6-96231b3b80d8
Fix broken test.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@196756 91177308-0d34-0410-b5e6-96231b3b80d8