Add a "wrapper" header that includes firestore_exceptions.h from the iOS repo (#538)

This simplifies the Unity build (the SWIG part of the build has issues with directly including a header that comes from the iOS repo).

Author: var-const

firestore/CMakeLists.txt

@@ -32,6 +32,7 @@ set(common_SRCS
     src/common/field_path.cc
     src/common/field_value.cc
     src/common/firestore.cc
+    src/common/firestore_exceptions_common.h
     src/common/futures.cc
     src/common/futures.h
     src/common/hard_assert_common.cc

firestore/src/common/firestore_exceptions_common.h

@@ -0,0 +1,22 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef FIREBASE_FIRESTORE_SRC_COMMON_FIRESTORE_EXCEPTIONS_COMMON_H_
+#define FIREBASE_FIRESTORE_SRC_COMMON_FIRESTORE_EXCEPTIONS_COMMON_H_
+
+// Note: this header only exists to simplify the Unity build (the SWIG build has
+// trouble directly including a header that comes from the iOS repo).
+#include "Firestore/core/src/util/firestore_exceptions.h"
+
+#endif  // FIREBASE_FIRESTORE_SRC_COMMON_FIRESTORE_EXCEPTIONS_COMMON_H_