Use std::unique_ptr to handle ownership of DwarfUnits in DwarfFile.

So Chandler - how about those range algorithms? (would really love a
dereferencing range adapter for this sort of stuff)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206921 91177308-0d34-0410-b5e6-96231b3b80d8