Reapply r202420 hopefully fixed for other STLs

Keep the copy constructor around, and add a FIXME that we should really
remove it as soon as we have C++11 std::map's emplace function.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202439 91177308-0d34-0410-b5e6-96231b3b80d8