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, 1 insertions, 1 deletions

diff --git a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
index 66c3c87..0e1da31 100644
--- a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
+++ b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
@@ -78,7 +78,7 @@ class FirebaseCredentialsProvider : public CredentialsProvider {
    */
   struct Contents {
     Contents(FIRApp* app, User&& user)
-        : app(app), current_user(std::move(user)), mutex() {
+        : app(app), current_user(std::move(user)) {
     }
 
     const FIRApp* app;