Fix a broken typedef; NFCI

Richard Smith noted that `typedef typename iplist::iplist_impl_type
iplist_impl_type` is incorrect, per
http://eel.is/c++draft/basic.scope#class-2

It seems that neither clang nor gcc get too angry about this, but a
newer version of msvc does.

Thanks to jcmac on IRC for pointing this out!

llvm-svn: 330639