Move some logic to  ModuleLinker::shouldLinkFromSource. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217449 91177308-0d34-0410-b5e6-96231b3b80d8
Make this input file pass the verifier.

This was not noticed before because llvm-link only runs the verifier on the
result and these globals were not present in the result.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217450 91177308-0d34-0410-b5e6-96231b3b80d8
When merging two common GlobalValues, keep the largest.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217451 91177308-0d34-0410-b5e6-96231b3b80d8