Add lint checking for Objective-C++ sources (#1048)

* lint.sh now lints Objective-C++ too

* cpplint checks system-style includes that should be user-style

This prevents it from recognizing project sources as if they were C
system headers and then complaining that they're in the wrong place.

* cpplint checks #imports and #includes

* cpplint checks that C++ system headers aren't #imported

* cpplint checks for C system headers that could be C++ system headers

* cpplint checks that Objective-C sources include their headers

1 files changed, 3 insertions, 2 deletions

diff --git a/Firestore/Source/Local/FSTLevelDB.mm b/Firestore/Source/Local/FSTLevelDB.mm
index 4812228..fae85e7 100644
--- a/Firestore/Source/Local/FSTLevelDB.mm
+++ b/Firestore/Source/Local/FSTLevelDB.mm
@@ -16,7 +16,7 @@
 
 #import "Firestore/Source/Local/FSTLevelDB.h"
 
-#include <leveldb/db.h>
+#include <memory>
 
 #import "FIRFirestoreErrors.h"
 #import "Firestore/Source/Local/FSTLevelDBMigrations.h"
@@ -26,13 +26,14 @@
 #import "Firestore/Source/Remote/FSTSerializerBeta.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 #import "Firestore/Source/Util/FSTLogger.h"
-#include "absl/memory/memory.h"
 
 #include "Firestore/core/src/firebase/firestore/auth/user.h"
 #include "Firestore/core/src/firebase/firestore/core/database_info.h"
 #include "Firestore/core/src/firebase/firestore/local/leveldb_transaction.h"
 #include "Firestore/core/src/firebase/firestore/model/database_id.h"
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
+#include "absl/memory/memory.h"
+#include "leveldb/db.h"
 
 namespace util = firebase::firestore::util;
 using firebase::firestore::auth::User;