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

1 files changed, 3 insertions, 2 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/field_value.cc b/Firestore/core/src/firebase/firestore/model/field_value.cc
index 1a40331..b0519f7 100644
--- a/Firestore/core/src/firebase/firestore/model/field_value.cc
+++ b/Firestore/core/src/firebase/firestore/model/field_value.cc
@@ -38,7 +38,7 @@ namespace {
 /**
  * This deviates from the other platforms that define TypeOrder. Since
  * we already define Type for union types, we use it together with this
- * function to achive the equivalent order of types i.e.
+ * function to achieve the equivalent order of types i.e.
  *     i) if two types are comparable, then they are of equal order;
  *    ii) otherwise, their order is the same as the order of their Type.
  */
@@ -148,7 +148,8 @@ FieldValue& FieldValue::operator=(FieldValue&& value) {
       return *this;
     default:
       // We just copy over POD union types.
-      return *this = value;
+      *this = value;
+      return *this;
   }
 }