Recommit r231175: Change LiveStackAnalysis::SS2IntervalMap from std::map to std:...

Recommit r231175: Change LiveStackAnalysis::SS2IntervalMap from std::map to std::unordered_map

The order of this container was needed at one point - so, at that point
create a temporary array of pointers, sort those, then iterate them.
This keeps lookup efficient (& the lesser issue, of allowing the use of
emplace... ), object identity preserved, and ordered iteration in the
one place that requires it.

While this has no functional change, I realize it does mean allocating
an extra data structure and performing a sort - so if this looks suspect
to anyone regarding perf characteristics, I'm all ears.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231189 91177308-0d34-0410-b5e6-96231b3b80d8