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...Nodes never “move around” in memory due to late uniquing etc, which means that TrackingVH shouldn’t be necessary, right? This should make all frontends a lot more memory efficient because they can just use raw pointers to MDNodes everywhere. Almost. Two caveats: 1. Handles should generally be `MDRef` instead of `Metadata*`, since I threw in reference-counting semantics so that no-longer-referenced metadata cleans itself up. 2. If a handle might point to a forward reference -- i.e., a `TempMDNode` in this patch -- it should use `TrackingMDRef`. When the forward reference gets...

TL;DR: If you use metadata (especially if it's out-of-tree), check the numbered list of lost functionality below to see whether I'm trying to break your compiler permanently. In response to my recent commits (e.g., [1]) that changed API from `MDNode` to `Value`, Eric had a really interesting idea [2] -- split metadata entirely from the `Value` hierarchy, and drop general support for