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, 4 insertions, 5 deletions

diff --git a/Firestore/Example/Tests/Local/FSTLevelDBRemoteDocumentCacheTests.mm b/Firestore/Example/Tests/Local/FSTLevelDBRemoteDocumentCacheTests.mm
index d69e613..7baf1ef 100644
--- a/Firestore/Example/Tests/Local/FSTLevelDBRemoteDocumentCacheTests.mm
+++ b/Firestore/Example/Tests/Local/FSTLevelDBRemoteDocumentCacheTests.mm
@@ -14,16 +14,15 @@
  * limitations under the License.
  */
 
-#import "Firestore/Example/Tests/Local/FSTRemoteDocumentCacheTests.h"
-
-#include <leveldb/db.h>
+#include <string>
 
+#import "Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h"
+#import "Firestore/Example/Tests/Local/FSTRemoteDocumentCacheTests.h"
 #import "Firestore/Source/Local/FSTLevelDB.h"
 #import "Firestore/Source/Local/FSTLevelDBKey.h"
 
-#import "Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h"
-
 #include "Firestore/core/src/firebase/firestore/util/ordered_code.h"
+#include "leveldb/db.h"
 
 NS_ASSUME_NONNULL_BEGIN