Rollup merge of #151577 - Zalathar:dep-kind-vtable, r=Kivooeo

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)

Author: matthiaskrgr