Revert "Use std::map<K, V> rather than std::map<K, std::unique_ptr<V>>"

Seems libstdc++ on some buildbots is lacking std::map::emplace, which is
weird... reverting while I look into it.

This reverts commit r222937.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222939 91177308-0d34-0410-b5e6-96231b3b80d8