[clang][dataflow] Weaken guard to only check for storage location

[li.zhe.hua]

Summary

Weaken the guard for whether a sub-expression has been evaluated to
only check for the storage location, instead of checking for the
value. It should be sufficient to check for the storage location, as
we don't necessarily guarantee that a value will be set for the
location (although this is currently true right now).

clang/lib/Analysis/FlowSensitive/TypeErasedDataflowAnalysis.cpp

void VisitConditionalOperator(const ConditionalOperator *S) {
     auto *Cond = S->getCond();
     assert(Cond != nullptr);
     extendFlowCondition(*Cond);
   }
 
 private:
   void extendFlowCondition(const Expr &Cond) {
     // The terminator sub-expression might not be evaluated.
-    if (Env.getValue(Cond, SkipPast::None) == nullptr)
+    if (Env.getStorageLocation(Cond, SkipPast::None) == nullptr)
       transfer(StmtToEnv, Cond, Env);
 
     // FIXME: The flow condition must be an r-value, so `SkipPast::None` should
     // suffice.
     auto *Val =
         cast_or_null<BoolValue>(Env.getValue(Cond, SkipPast::Reference));
     // Value merging depends on flow conditions from different environments
     // being mutually exclusive -- that is, they cannot both be true in their