Add clang-tidy checks for Firestore (#1078)

* Add a .clang-tidy configuration for Firestore C++

* Fix clang-tidy warnings

  * typedef -> using
  * const ref + rvalue ref -> pass by value
  * NULL -> nullptr
  * remove useless default initializations
  * remove useless const value-type parameter declarations (definitions
    can still use them)
  * use auto instead of repeating types in a cast

* Fix typos

* Address use of static method through instance warnings

* Address use after move warnings

3 files changed, 4 insertions, 3 deletions

diff --git a/Firestore/core/test/firebase/firestore/immutable/array_sorted_map_test.cc b/Firestore/core/test/firebase/firestore/immutable/array_sorted_map_test.cc
index fceab7d..6758dd5 100644
--- a/Firestore/core/test/firebase/firestore/immutable/array_sorted_map_test.cc
+++ b/Firestore/core/test/firebase/firestore/immutable/array_sorted_map_test.cc
@@ -29,7 +29,7 @@ namespace firestore {
 namespace immutable {
 namespace impl {
 
-typedef ArraySortedMap<int, int> IntMap;
+using IntMap = ArraySortedMap<int, int>;
 constexpr IntMap::size_type kFixedSize = IntMap::kFixedSize;
 
 // TODO(wilhuff): ReverseTraversal
@@ -153,7 +153,7 @@ TEST(ArraySortedMap, EmptyRemoval) {
 
 TEST(ArraySortedMap, InsertionAndRemovalOfMaxItems) {
   auto expected_size = kFixedSize;
-  int n = static_cast<int>(expected_size);
+  auto n = static_cast<int>(expected_size);
   std::vector<int> to_insert = Shuffled(Sequence(n));
   std::vector<int> to_remove = Shuffled(to_insert);
 
diff --git a/Firestore/core/test/firebase/firestore/immutable/sorted_map_test.cc b/Firestore/core/test/firebase/firestore/immutable/sorted_map_test.cc
index 44dca50..747c66b 100644
--- a/Firestore/core/test/firebase/firestore/immutable/sorted_map_test.cc
+++ b/Firestore/core/test/firebase/firestore/immutable/sorted_map_test.cc
@@ -49,6 +49,7 @@ class SortedMapTest : public ::testing::Test {
   }
 };
 
+// NOLINTNEXTLINE: must be a typedef for the gtest macros
 typedef ::testing::Types<SortedMap<int, int>,
                          impl::ArraySortedMap<int, int>,
                          impl::TreeSortedMap<int, int>>
diff --git a/Firestore/core/test/firebase/firestore/immutable/tree_sorted_map_test.cc b/Firestore/core/test/firebase/firestore/immutable/tree_sorted_map_test.cc
index 7a96b67..c03dc6c 100644
--- a/Firestore/core/test/firebase/firestore/immutable/tree_sorted_map_test.cc
+++ b/Firestore/core/test/firebase/firestore/immutable/tree_sorted_map_test.cc
@@ -25,7 +25,7 @@ namespace firestore {
 namespace immutable {
 namespace impl {
 
-typedef TreeSortedMap<int, int> IntMap;
+using IntMap = TreeSortedMap<int, int>;
 
 TEST(TreeSortedMap, EmptySize) {
   IntMap map;