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

2 files changed, 2 insertions, 3 deletions

diff --git a/Firestore/core/src/firebase/firestore/immutable/sorted_map_base.h b/Firestore/core/src/firebase/firestore/immutable/sorted_map_base.h
index accb5ef..cfb19c1 100644
--- a/Firestore/core/src/firebase/firestore/immutable/sorted_map_base.h
+++ b/Firestore/core/src/firebase/firestore/immutable/sorted_map_base.h
@@ -17,7 +17,7 @@
 #ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_SORTED_MAP_BASE_H_
 #define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_SORTED_MAP_BASE_H_
 
-#include <stdint.h>
+#include <cstdint>
 
 namespace firebase {
 namespace firestore {
diff --git a/Firestore/core/src/firebase/firestore/immutable/tree_sorted_map.h b/Firestore/core/src/firebase/firestore/immutable/tree_sorted_map.h
index e3102e7..dfe270d 100644
--- a/Firestore/core/src/firebase/firestore/immutable/tree_sorted_map.h
+++ b/Firestore/core/src/firebase/firestore/immutable/tree_sorted_map.h
@@ -17,9 +17,8 @@
 #ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_TREE_SORTED_MAP_H_
 #define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_IMMUTABLE_TREE_SORTED_MAP_H_
 
-#include <assert.h>
-
 #include <algorithm>
+#include <cassert>
 #include <functional>
 #include <memory>
 #include <utility>