Fix Darwin linker. Patch from Jean-Daniel Dupas.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223865 91177308-0d34-0410-b5e6-96231b3b80d8
Re-commit r223330: Rewrite InputGraph's Group

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223867 91177308-0d34-0410-b5e6-96231b3b80d8