Merge ELFGroup with Group.

Group class is designed for GNU LD's --start-group and --end-group. There's
no obvious need to have two classes for it -- one as an abstract base class
and the other as a concrete class.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205375 91177308-0d34-0410-b5e6-96231b3b80d8