Rust: add DB upgrade/downgrade scripts and change note for ra_ap 0.0.347

The schema delta for ra_ap 0.0.347 adds the new node kinds DerefPat,
ImplRestriction, IncludeBytesExpr, MutRestriction, NotNull, PatternTypeRepr and
VisibilityInner, moves a Visibility path onto the new VisibilityInner, replaces
the text-less FormatArgsArgName placeholder with a Name, and adds attrs to the
inline assembly nodes, mut_restriction to Struct/TupleField and impl_restriction
to Trait.

The upgrade repurposes the old format-arg placeholder ids as Name ids (avoiding
dangling refs) and synthesises a VisibilityInner per visibility with a path. The
downgrade rejoins the visibility path, repurposes ids back to the placeholder,
and scrubs the locations of the deleted node kinds. compatibility is partial:
the new node kinds are empty on old DBs, so a rebuild yields strictly-better
results while existing queries keep working.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 890cb55d-0437-4bd7-9268-87b8897ada01

Author: redsun82

rust/downgrades/852696f20117c338fbb4e6a4f5ed69df79682c37/downgrade.ql

@@ -0,0 +1,50 @@
+class Element extends @element {
+  string toString() { none() }
+}
+
+class Location extends @location_default {
+  string toString() { none() }
+}
+
+// Genuinely-new node kinds with no representation in the old schema. Their own child relations are
+// dropped via `delete` in upgrade.properties; here we additionally drop their locations so no
+// dangling `locatable_locations` rows remain.
+private predicate deletedElement(Element id) {
+  deref_pats(id) or
+  not_nulls(id) or
+  include_bytes_exprs(id) or
+  pattern_type_reprs(id) or
+  impl_restrictions(id) or
+  mut_restrictions(id) or
+  visibility_inners(id)
+}
+
+// A `@name` used as a format argument's name. The old schema represents these as dedicated text-less
+// `@format_args_arg_name` placeholders, so we repurpose these ids into that entity table and drop
+// them (and their text) from `names`/`name_texts`.
+private predicate formatArgName(Element name) { format_args_arg_names(_, name) }
+
+// The new schema inserts a `VisibilityInner` node between `Visibility` and its path; the old schema
+// stores the path directly on the `Visibility`, so we rejoin the two hops.
+query predicate new_visibility_paths(Element visibility, Element path) {
+  exists(Element inner |
+    visibility_visibility_inners(visibility, inner) and
+    visibility_inner_paths(inner, path)
+  )
+}
+
+query predicate new_format_args_arg_names(Element id) { formatArgName(id) }
+
+query predicate new_format_args_arg_arg_names(Element arg, Element name) {
+  format_args_arg_names(arg, name)
+}
+
+query predicate new_names(Element id) { names(id) and not formatArgName(id) }
+
+query predicate new_name_texts(Element id, string text) {
+  name_texts(id, text) and not formatArgName(id)
+}
+
+query predicate new_locatable_locations(Element id, Location location) {
+  locatable_locations(id, location) and not deletedElement(id)
+}