C++: Add getDebugString predicate and use it in ValueNumberPropertyProvider

Author: MathiasVP

cpp/ql/src/semmle/code/cpp/ir/internal/ASTValueNumbering.qll

@@ -61,6 +61,8 @@ class GVN extends TValueNumber {
 
   final string toString() { result = "GVN" }
 
+  final string getDebugString() { result = strictconcat(getAnExpr().toString(), ", ") }
+
   final Location getLocation() {
     result =
       min(Location l |