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
Remove unused enum Position::ANY and third parameter of insertElementAt().

insertElementAt()'s third parameter is not only unused but also ignored
if you pass Position::END. The actual meaning of the parameter was obscure.

Differential Revision: http://llvm-reviews.chandlerc.com/D3256

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205376 91177308-0d34-0410-b5e6-96231b3b80d8
De-virtualize InputGraph's member functions.

There's no class derived from InputGraph. Making member functions virtual
just makes it a bit inefficient.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@205377 91177308-0d34-0410-b5e6-96231b3b80d8