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, 1 deletions

diff --git a/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm b/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm
index b228657..5b5bdd1 100644
--- a/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm
+++ b/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm
@@ -17,6 +17,9 @@
 #import "Firestore/Example/Tests/Local/FSTMutationQueueTests.h"
 
 #import <FirebaseFirestore/FIRTimestamp.h>
+
+#include <set>
+
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Local/FSTEagerGarbageCollector.h"
 #import "Firestore/Source/Local/FSTMutationQueue.h"
@@ -246,7 +249,6 @@ NS_ASSUME_NONNULL_BEGIN
         return newBatches;
       });
   self.persistence.run("testNextMutationBatchAfterBatchIDSkipsAcknowledgedBatches", [&]() {
-
     [self.mutationQueue acknowledgeBatch:batches[0] streamToken:nil];
     XCTAssertEqualObjects([self.mutationQueue nextMutationBatchAfterBatchID:kFSTBatchIDUnknown],
                           batches[1]);