A DenseMap of a std::map isn't a very good idea because the "grow()" method will

A DenseMap of a std::map isn't a very good idea because the "grow()" method will
need to make a deep copy of each of the std::maps. Use a std::map of the
std::map instead. This improves the compile time of sqlite3 by ~2%.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@148003 91177308-0d34-0410-b5e6-96231b3b80d8