Simplify ModuleLinker::getLinkageResult. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217441 91177308-0d34-0410-b5e6-96231b3b80d8
Prefer common over weak linkage when linking.

This matches the behavior of ELF linkers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217443 91177308-0d34-0410-b5e6-96231b3b80d8