Revert "Rewrite InputGraph's Group"

This reverts commit r223330 because it broke Darwin and ELF
linkers in a way that we couldn't have caught with the existing
test cases.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@223373 91177308-0d34-0410-b5e6-96231b3b80d8