These functions are definitely there in 4.17 Preview 2.
This functionality is referenced in the 4.17 preview thread here: Unreal Engine 4.17 Preview - Announcements - Epic Developer Community Forums
The Asset Manager now has Blueprint access and is ready for production. The Asset Manager is a global object that can be used to discover, load, and audit Maps and game-specific asset types in the editor or at runtime. It provides a framework to make it easier to create things like Quests, Weapons, or Heroes and load them on demand, and can also be used to set up cook and chunk rules when packaging and releasing your game. The Asset Manager tab in Project Settings can be used to set up the rules for your game. Primary Asset Types that are scanned by the Asset Manager can be queried at runtime before they are loaded, and can then be asynchronously loaded on demand. To use the asset manager, call functions on the native UAssetManager class (which can be subclassed for your game), or call Blueprint functions in the Asset Manager category such as Async Load Primary Asset.
The ‘ToString’ and ‘FromString’ functions for FPrimaryAssetId in C++ are missing from this Blueprint Access feature.