Revert "Fix crash from accessing a Check::Context during lowering (#7335)" (#7353)

This reverts commit afd679129d.

The commit introduced ASAN errors:

https://github.com/carbon-language/carbon-lang/actions/runs/27436787960/job/81100665405

toolchain/check/cpp/export.cpp

@@ -177,6 +177,39 @@ auto ExportClassToCpp(Context& context, SemIR::LocId loc_id,
   return record_decl;
 }
 
+// Get the `StructTypeField`s from a class's object repr.
+static auto GetStructTypeFields(Context& context,
+                                const SemIR::Class& class_info)
+    -> llvm::ArrayRef<SemIR::StructTypeField> {
+  if (class_info.adapt_id.has_value()) {
+    // The representation of an adapter won't necessarily be a
+    // struct. Return an empty array since adapters can't declare
+    // fields.
+    return {};
+  }
+
+  auto object_repr_type_id =
+      class_info.GetObjectRepr(context.sem_ir(), SemIR::SpecificId::None);
+  if (object_repr_type_id == SemIR::ErrorInst::TypeId) {
+    return {};
+  }
+  auto struct_type =
+      context.types().GetAs<SemIR::StructType>(object_repr_type_id);
+  return context.struct_type_fields().Get(struct_type.fields_id);
+}
+
+static auto LookupClassFieldByStructField(
+    const Context& context, const SemIR::NameScope& class_scope,
+    const SemIR::StructTypeField& struct_field)
+    -> std::optional<SemIR::InstStore::GetAsWithIdResult<SemIR::FieldDecl>> {
+  if (auto entry_id = class_scope.Lookup(struct_field.name_id)) {
+    auto field_inst_id =
+        class_scope.GetEntry(*entry_id).result.target_inst_id();
+    return context.insts().TryGetAsWithId<SemIR::FieldDecl>(field_inst_id);
+  }
+  return std::nullopt;
+}
+
 static auto SetCppClassMemberAccess(const SemIR::NameScope& class_scope,
                                     SemIR::NameId member_name_id,
                                     clang::Decl* member) -> void {
@@ -232,10 +265,9 @@ auto ExportAllFieldsToCpp(Context& context, SemIR::Class& class_info) -> void {
 
   const auto& class_scope = context.name_scopes().Get(class_info.scope_id);
 
-  for (const auto& struct_field :
-       class_info.GetStructTypeFields(context.sem_ir())) {
-    auto class_field = LookupClassFieldByStructField(context.sem_ir(),
-                                                     class_scope, struct_field);
+  for (const auto& struct_field : GetStructTypeFields(context, class_info)) {
+    auto class_field =
+        LookupClassFieldByStructField(context, class_scope, struct_field);
     if (!class_field) {
       continue;
     }
@@ -282,6 +314,42 @@ auto ExportFieldToCpp(Context& context, SemIR::InstId field_inst_id,
   return nullptr;
 }
 
+auto CalculateCppFieldOffsets(
+    Context& context, SemIR::ClassId class_id,
+    llvm::DenseMap<const clang::FieldDecl*, uint64_t>& field_offsets) -> bool {
+  auto class_info = context.classes().Get(class_id);
+  const auto& class_scope = context.name_scopes().Get(class_info.scope_id);
+
+  auto class_layout = SemIR::ObjectLayout::Empty();
+  for (const auto& struct_field : GetStructTypeFields(context, class_info)) {
+    auto field_type_id = context.sem_ir().types().GetTypeIdForTypeInstId(
+        struct_field.type_inst_id);
+    auto field_layout = context.sem_ir()
+                            .types()
+                            .GetCompleteTypeInfo(field_type_id)
+                            .object_layout;
+
+    // Use the field's name to look up the corresponding entry in the
+    // class. If it's a `FieldDecl`, write out the offset of the
+    // corresponding `clang::FieldDecl`.
+    auto class_field =
+        LookupClassFieldByStructField(context, class_scope, struct_field);
+    if (class_field) {
+      auto* cpp_field_decl =
+          ExportFieldToCpp(context, class_field->inst_id, class_field->inst);
+      if (!cpp_field_decl) {
+        return false;
+      }
+      field_offsets.insert(
+          {cpp_field_decl, class_layout.FieldOffset(field_layout).bits()});
+    }
+
+    class_layout.AppendField(field_layout);
+  }
+
+  return true;
+}
+
 namespace {
 struct FunctionInfo {
   struct Param {