Include an "implementation" if SGI's slist. This was quickly hacked

together to get it working with code, and is neither optimal
(erase(Iterator, Iterator) calculates the previous iterator twice,
rather than calculating the previous iterator of the first one, then
advancing it until the second is found) nor complete (splice() was not
implemented). Most of the implementation is borrowed from forward_list
via using-declarations.

llvm-svn: 136542