Use an ilist instead of std::list. NFC.

The TrieNode/TrieEdge data structures here are allocated in a bumpptrallocator.

Unfortunately, TrieNode contained a std::list<TrieEdge> and as the allocator doesn't
call the TrieNode destructor, we ended up leaking the memory allocated by the std::list
itself.

Instead we can use an intrusive list as then we save the extra allocations anyway.

llvm-svn: 258725