firebase-ios-sdk

Fix Firestore compilation under Xcode < 9.2 (#1367)

* Don't rely on specialization failure to determine when std::hash is unavailable.

Instead manually declare the conditions under which std::hash should be
defined.

* Fix detection of Objective-C classes in Xcode < 9.2

std::is_base_of<NSObject, NSString>{} is false there so the overloads
defined for Objective-C types weren't getting enabled.

* Add explicit tests for StringFormat using Objective-C objects

* Add explicit tests for HasStdHash