[dataflow] handle missing case in value debug strings

clang/lib/Analysis/FlowSensitive/DebugSupport.cpp

#endif
   /// Returns a string representation of a boolean value `B`.
   std::string debugString(const BoolValue &B, size_t Depth = 0) {
     std::string S;
     switch (B.getKind()) {
     case Value::Kind::AtomicBool: {
       S = getAtomName(&cast<AtomicBoolValue>(B));
       break;
     }
+    case Value::Kind::TopBool: {
+      S = "top";
+      break;
+    }
     case Value::Kind::Conjunction: {
       auto &C = cast<ConjunctionValue>(B);
       auto L = debugString(C.getLeftSubValue(), Depth + 1);
       auto R = debugString(C.getRightSubValue(), Depth + 1);
       S = formatv("(and\n{0}\n{1})", L, R);
       break;
     }
     case Value::Kind::Disjunction: {

clang/unittests/Analysis/FlowSensitive/DebugSupportTest.cpp

 using testing::StrEq;
 
 TEST(BoolValueDebugStringTest, AtomicBoolean) {
   // B0
   ConstraintContext Ctx;
   auto B = Ctx.atom();
 
   auto Expected = R"(B0)";
-  debugString(*B);
+  EXPECT_THAT(debugString(*B), StrEq(Expected));
+}
+
+TEST(BoolValueDebugStringTest, Top) {
+  ConstraintContext Ctx;
+  auto B = Ctx.top();
+
+  auto Expected = R"(top)";
   EXPECT_THAT(debugString(*B), StrEq(Expected));
 }
 
 TEST(BoolValueDebugStringTest, Negation) {
   // !B0
   ConstraintContext Ctx;
   auto B = Ctx.neg(Ctx.atom());