[clang][dataflow] Perform structural comparison of indirection values in join.

This patch changes `Environment::join`, in the case that two values at the same
location are not (pointer) equal, to structurally compare indirection values
(pointers and references) for equivalence (that is, equivalent pointees) before
resorting to merging.

This change makes join consistent with equivalence, which also performs
structural comparison. It also fixes a bug where the values are `ReferenceValue`
but the merge creates a non-reference value. This case arises when the
`ReferenceValue`s were created to represent an lvalue, so the "reference-ness"
is not reflected in the type. In this case, the pointees will always be
equivalent, because lvalues at the same code location point to the location of a
fixed declaration, whose location is itself stable across blocks.

We were unable to reproduce a unit test for this latter bug, but have verified
the fix in the context of a larger piece of code that triggers the bug.

Differential Revision: https://reviews.llvm.org/D124540

Author: ymand

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

@@ -21,6 +21,7 @@
 #include "clang/Analysis/FlowSensitive/Value.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/DenseSet.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
 #include <memory>
@@ -105,6 +106,15 @@ static Value *mergeDistinctValues(QualType Type, Value *Val1,
     return &MergedEnv.makeOr(*Expr1, *Expr2);
   }
 
+  // FIXME: add unit tests that cover this statement.
+  if (auto *IndVal1 = dyn_cast<IndirectionValue>(Val1)) {
+    auto *IndVal2 = cast<IndirectionValue>(Val2);
+    assert(IndVal1->getKind() == IndVal2->getKind());
+    if (&IndVal1->getPointeeLoc() == &IndVal2->getPointeeLoc()) {
+      return Val1;
+    }
+  }
+
   // FIXME: Consider destroying `MergedValue` immediately if `ValueModel::merge`
   // returns false to avoid storing unneeded values in `DACtx`.
   if (Value *MergedVal = MergedEnv.createValue(Type))