Implement a !foreach operator analogous to GNU make's $(foreach).

Implement a !foreach operator analogous to GNU make's $(foreach).
Use it on dags and lists like this:

class decls {
  string name;
}

def Decls : decls;

class B<list<string> names> : A<!foreach(Decls.name, names, !strconcat(Decls.name, ", Sr."))>;

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@71803 91177308-0d34-0410-b5e6-96231b3b80d8