Swift: add upgrade/downgrade scripts

swift/downgrades/15a630f68e14f053932cf6a23797f43d958eedc9/downgrade.ql

@@ -0,0 +1,18 @@
+class Element extends @element {
+  string toString() { none() }
+}
+
+query predicate new_unspecified_elements(Element e, string property, string error) {
+  unspecified_elements(e, property, error)
+  or
+  error =
+    "ThenStmt nodes removed during database downgrade. Please update your CodeQL code." and
+  property = "" and
+  then_stmts(e, _)
+}
+
+query predicate new_unspecified_element_children(Element e, int index, Element child) {
+  unspecified_element_children(e, index, child)
+  or
+  then_stmts(e, child) and index = 0
+}

swift/downgrades/15a630f68e14f053932cf6a23797f43d958eedc9/upgrade.properties

@@ -0,0 +1,5 @@
+description: Remove `ThenStmt` wrapper nodes.
+compatibility: partial
+unspecified_elements.rel: run downgrade.ql new_unspecified_elements
+unspecified_element_children.rel: run downgrade.ql new_unspecified_element_children
+then_stmts.rel: delete

swift/ql/lib/upgrades/60be249ad164f6e4b43c203323f1b3956dc97c2f/upgrade.properties

@@ -0,0 +1,2 @@
+description: Add `ThenStmt` wrapper nodes.
+compatibility: full