Sema: Replace the SetVector/DenseMap/std::sort combination with a simple std::map

Sema: Replace the SetVector/DenseMap/std::sort combination with a simple std::map

This guarantees the order and doesn't increase malloc counts a lot as there are
typically very few elements int the map. Provide a little iterator adapter to
keep the same interface as we had with the flat sorted list.

No functional change intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@232173 91177308-0d34-0410-b5e6-96231b3b80d8