This serves as an index into arrays built by make_dep_kind_array.
A dep-node filter goes from a user-defined string to a query over
nodes. Right now the format is like this:
A filter like F -> G where F and G are valid dep-node
filters. This can be used to test the source/target independently.
Data for use when recompiling the current crate.
A “work product” is an intermediate result that we save into the
incremental directory for later re-use. The primary example are
the object files that we save for each partition at code
generation time.
A “work product” corresponds to a .o (or other) file that we
save in between runs. These IDs do not have a DefId but rather
some independent path or string that persists between runs without
the need to be mapped or unmapped. (This ensures we can serialize
them even in the absence of a tcx.)