[PECOFF] Do not create undefined atom for common symbol.

We were creating undefined atoms for common symbols by mistake. That did not
lead to a link failure, for undefined atoms would be resolved by common symbols
in the same file, but that's a waste of resource.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@189534 91177308-0d34-0410-b5e6-96231b3b80d8