[clang] [analyzer] Only report the first dereference of the same variable in DanglingPtrDeref (PR #215409)

[Balázs Benics via cfe-commits]

================
@@ -86,6 +87,13 @@ bool lifetime_modeling::isDeallocated(ProgramStateRef State,
   return State->contains<DeallocatedSourceSet>(Region->getBaseRegion());
 }
 
+ProgramStateRef lifetime_modeling::markAsReported(ProgramStateRef State,
+                                                  const MemRegion *Region) {
+  if (State->contains<ReportedDeadRegions>(Region->getBaseRegion()))
+    return nullptr;
+  return State->add<ReportedDeadRegions>(Region->getBaseRegion());
----------------
steakhal wrote:

This is the usual double lookup problem. This is my pet peeve.
`add` is idempotent (identity) if the thing you wanna put into was already there. You could add unconditionally and check if the resulting state is different. If so, the addition succeeded. If not, then return `nullptr`.

https://github.com/llvm/llvm-project/pull/215409