Replace the `std::vector<>` for `DIE::Children` with an intrusively
linked list. This is a strict memory improvement: it requires no
auxiliary storage, and reduces `sizeof(DIE)` by one pointer. It also
factors out the DIE-related malloc traffic.
This drops llc memory usage from 735 MB down to 718 MB, or ~2.3%.
(I'm looking at `llc` memory usage on `verify-uselistorder.lto.opt.bc`;
see r236629 for details.)
llvm-svn: 240736
827200c822