AsmPrinter: Use an intrusively linked list for DIE::Children

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.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240736 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DwarfFile.cpp

@@ -111,7 +111,7 @@ unsigned DwarfFile::computeSizeAndOffset(DIE &Die, unsigned Offset) {
     assert(Abbrev.hasChildren() && "Children flag not set");
 
     for (auto &Child : Die.children())
-      Offset = computeSizeAndOffset(*Child, Offset);
+      Offset = computeSizeAndOffset(Child, Offset);
 
     // End of children marker.
     Offset += sizeof(int8_t);