[clang][dataflow] Handle AtomicExpr in ResultObjectVisitor. (#94963)

This is one of the node kinds that should be considered an "original
initializer". The patch adds a test that was causing an assertion
failure in
`assert(Children.size() == 1)` without the fix.

clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp

@@ -415,7 +415,7 @@ class ResultObjectVisitor : public AnalysisASTVisitor<ResultObjectVisitor> {
     // below them can initialize the same object (or part of it).
     if (isa<CXXConstructExpr>(E) || isa<CallExpr>(E) || isa<LambdaExpr>(E) ||
         isa<CXXDefaultArgExpr>(E) || isa<CXXDefaultInitExpr>(E) ||
-        isa<CXXStdInitializerListExpr>(E) ||
+        isa<CXXStdInitializerListExpr>(E) || isa<AtomicExpr>(E) ||
         // We treat `BuiltinBitCastExpr` as an "original initializer" too as
         // it may not even be casting from a record type -- and even if it is,
         // the two objects are in general of unrelated type.

clang/unittests/Analysis/FlowSensitive/TransferTest.cpp

@@ -3345,6 +3345,32 @@ TEST(TransferTest, ResultObjectLocationForBuiltinBitCastExpr) {
       });
 }
 
+TEST(TransferTest, ResultObjectLocationForAtomicExpr) {
+  std::string Code = R"(
+    struct S {};
+    void target(_Atomic(S) *ptr) {
+      S s = __c11_atomic_load(ptr, __ATOMIC_SEQ_CST);
+      // [[p]]
+    }
+  )";
+  using ast_matchers::atomicExpr;
+  using ast_matchers::match;
+  using ast_matchers::selectFirst;
+  using ast_matchers::traverse;
+  runDataflow(
+      Code,
+      [](const llvm::StringMap<DataflowAnalysisState<NoopLattice>> &Results,
+         ASTContext &ASTCtx) {
+        const Environment &Env = getEnvironmentAtAnnotation(Results, "p");
+
+        auto *Atomic = selectFirst<AtomicExpr>(
+            "atomic", match(atomicExpr().bind("atomic"), ASTCtx));
+
+        EXPECT_EQ(&Env.getResultObjectLocation(*Atomic),
+                  &getLocForDecl<RecordStorageLocation>(ASTCtx, Env, "s"));
+      });
+}
+
 TEST(TransferTest, ResultObjectLocationPropagatesThroughConditionalOperator) {
   std::string Code = R"(
     struct A {