Revert "[clang][dataflow] Correctly handle `InitListExpr` of union type." (#82856)

Reverts llvm/llvm-project#82348, which caused crashes when analyzing
empty InitListExprs for unions, e.g.

```cc
union U {
  double double_value;
  int int_value;
};

void target() {
  U value;
  value = {};
}
```

Co-authored-by: Samira Bazuzi <bazuzi@users.noreply.github.com>

1 files changed, 4 insertions, 14 deletions

diff --git a/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp b/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
index 0cfc26e..d487944 100644
--- a/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
+++ b/clang/lib/Analysis/FlowSensitive/DataflowEnvironment.cpp
@@ -361,8 +361,8 @@ getFieldsGlobalsAndFuncs(const Stmt &S, FieldSet &Fields,
     if (const auto *FD = dyn_cast<FieldDecl>(VD))
       Fields.insert(FD);
   } else if (auto *InitList = dyn_cast<InitListExpr>(&S)) {
-    if (InitList->getType()->isRecordType())
-      for (const auto *FD : getFieldsForInitListExpr(InitList))
+    if (RecordDecl *RD = InitList->getType()->getAsRecordDecl())
+      for (const auto *FD : getFieldsForInitListExpr(RD))
         Fields.insert(FD);
   }
 }
@@ -1104,22 +1104,12 @@ RecordStorageLocation *getBaseObjectLocation(const MemberExpr &ME,
   return Env.get<RecordStorageLocation>(*Base);
 }
 
-std::vector<const FieldDecl *>
-getFieldsForInitListExpr(const InitListExpr *InitList) {
-  const RecordDecl *RD = InitList->getType()->getAsRecordDecl();
-  assert(RD != nullptr);
-
-  std::vector<const FieldDecl *> Fields;
-
-  if (InitList->getType()->isUnionType()) {
-    Fields.push_back(InitList->getInitializedFieldInUnion());
-    return Fields;
-  }
-
+std::vector<FieldDecl *> getFieldsForInitListExpr(const RecordDecl *RD) {
   // Unnamed bitfields are only used for padding and do not appear in
   // `InitListExpr`'s inits. However, those fields do appear in `RecordDecl`'s
   // field list, and we thus need to remove them before mapping inits to
   // fields to avoid mapping inits to the wrongs fields.
+  std::vector<FieldDecl *> Fields;
   llvm::copy_if(
       RD->fields(), std::back_inserter(Fields),
       [](const FieldDecl *Field) { return !Field->isUnnamedBitfield(); });