Rename `DepKindStruct` to `DepKindVTable` This type is used by dependency-tracking code in the query system, for looking up function pointers and other metadata associated with a particular `DepKind`. Calling it “struct” is not particularly helpful, whereas calling it a “vtable” at least gives some basic intuition for what it is and how it is used. Some associated identifiers have also drifted a bit over time, and this PR adjusts those as well. There should be no change to compiler behaviour. r? nnethercote (or compiler)