[clang][dataflow] Strengthen flow condition assertions.

[mboehme]

Summary

Instead of asserting merely that the flow condition doesn't imply that a variable is true, make the stronger assertion that the flow condition implies that the variable is false.

[mboehme]

Adapted to Formula changes

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

runDataflow(
 
         const ValueDecl *BarDecl = findValueDecl(ASTCtx, "Bar");
         ASSERT_THAT(BarDecl, NotNull());
 
         auto &BarValThen = getFormula(*BarDecl, EnvThen);
         EXPECT_TRUE(EnvThen.flowConditionImplies(BarValThen));
 
         auto &BarValElse = getFormula(*BarDecl, EnvElse);
-        EXPECT_FALSE(EnvElse.flowConditionImplies(BarValElse));
+        EXPECT_TRUE(
+            EnvElse.flowConditionImplies(EnvElse.arena().makeNot(BarValElse)));
       });
 }
 
 TEST(TransferTest, BooleanInequality) {
   std::string Code = R"(
     void target(bool Bar) {
       bool Foo = true;
       if (Bar != Foo) {
runDataflow(
             getEnvironmentAtAnnotation(Results, "p-then");
         const Environment &EnvElse =
             getEnvironmentAtAnnotation(Results, "p-else");
 
         const ValueDecl *BarDecl = findValueDecl(ASTCtx, "Bar");
         ASSERT_THAT(BarDecl, NotNull());
 
         auto &BarValThen = getFormula(*BarDecl, EnvThen);
-        EXPECT_FALSE(EnvThen.flowConditionImplies(BarValThen));
+        EXPECT_TRUE(
+            EnvThen.flowConditionImplies(EnvThen.arena().makeNot(BarValThen)));
 
         auto &BarValElse = getFormula(*BarDecl, EnvElse);
         EXPECT_TRUE(EnvElse.flowConditionImplies(BarValElse));
       });
 }
 
 TEST(TransferTest, IntegerLiteralEquality) {
   std::string Code = R"(