[clang] [clang][dataflow] Fix crash on base-to-derived cast of unmodeled pointer value. (PR #179060)

[Artem Dergachev via cfe-commits]

haoNoQ wrote:

> (As usual, `BinaryConditionalOperator` comes to mind as the example of an expression that loads its sub-expressions multiple times. Let me see if I can break it.)

Yeah, for completeness, I guess it may theoretically be a problem in a test such as this one, in the context of `TypeErasedDataflowAnalysisTest.cpp` where there's this little `is_null` analysis:
```
TEST_F(TopTest, CastNeedsAValue) {
  std::string Code = R"(
    struct Base {};
    struct Derived: Base {};

    __attribute__((noreturn)) Derived *fail();

    Base *unknown();

    void target() {
      Derived *DPtr = static_cast<Derived *>(unknown()) ?: fail();
      // [[p]]
    }
  )";
  runDataflow(
      Code,
      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
         const AnalysisOutputs &AO) {
        AO.ACFG.getCFG().dump(AO.ASTCtx.getLangOpts(), true);
        ASSERT_THAT(Results.keys(), UnorderedElementsAre("p"));
        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
        Env.dump();

        const ValueDecl *DPtrDecl = findValueDecl(AO.ASTCtx, "DPtr");
        ASSERT_THAT(DPtrDecl, NotNull());

        EXPECT_EQ(Env.getValue(*DPtrDecl)->getProperty("is_null"),
                  &Env.getBoolLiteralValue(false));
      });
}
```
- where we wouldn't notice that `DPtr2` is non-null unless we eagerly assign a value to the static cast expression, because we'll be looking up the value of the operator `?:` from the next CFG block, which would be unconditionally equal to the condition value, which we've already forgot.

But all of this *would* only be a problem if we model the GNU binary form of operator `?:` in the first place. And it's not like we even model null checks. And even then, that's completely ridiculous code.

So, yeah, IIUC I can just skip the value. I'll get back to this tomorrow.

https://github.com/llvm/llvm-project/pull/179060