Fixes the MSVC build.

Commit r152704 exposed a latent MSVC limitation (aka bug). 
Both ilist and and iplist contains the same function:
  template<class InIt> void insert(iterator where, InIt first, InIt last) {
    for (; first != last; ++first) insert(where, *first);
  }

Also ilist inherits from iplist and ilist contains a "using iplist<NodeTy>::insert".
MSVC doesn't know which one to pick and complain with an error.

I think it is safe to delete ilist::insert since it is redundant anyway.

llvm-svn: 152746