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The remaining use of ConstString in StructuredData is the Dictionary class. Internally it's backed by a `std::map<ConstString, ObjectSP>`. I propose that we replace it with a `llvm::StringMap<ObjectSP>`. Many StructuredData::Dictionary objects are ephemeral and only exist for a short amount of time. Many of these Dictionaries are only produced once and are never used again. That leaves us with a lot of string data in the ConstString StringPool that is sitting there never to be used again. Even if the same string is used many times for keys of different Dictionary objects, that is something we can measure and adjust for instead of assuming that every key may be reused at some point in the future. Quick comparisons of key data is likely not a concern with Dictionary, but the use of `llvm::StringMap` means that lookups should be fast with its hashing strategy. Switching to a llvm::StringMap meant that the iteration order may be different. To account for this when serializing/dumping the dictionary, I added some code to sort the output by key before emitting anything. Differential Revision: https://reviews.llvm.org/D159313