C++: add upgrade and downgrade scripts

cpp/downgrades/73af5058c6899dcdb05754c27ca966aeb3a68c94/exprs.ql

@@ -0,0 +1,13 @@
+class Expr extends @expr {
+  string toString() { none() }
+}
+
+class Location extends @location_expr {
+  string toString() { none() }
+}
+
+from Expr expr, int kind, int kind_new, Location location
+where
+  exprs(expr, kind, location) and
+  if expr instanceof @blockassignexpr then kind_new = 0 else kind_new = kind
+select expr, kind_new, location

cpp/downgrades/73af5058c6899dcdb05754c27ca966aeb3a68c94/upgrade.properties

@@ -0,0 +1,3 @@
+description: Support block assignment
+compatibility: partial
+exprs.rel: run exprs.qlo

cpp/ql/lib/upgrades/34549c3b0937002f11037d01822ebe99442c1402/upgrade.properties

@@ -0,0 +1,2 @@
+description: Support block assignment
+compatibility: backwards