firebase-ios-sdk

Fix Firestore source errors under VS2017 (#1515)

* Project file updates from sync_project.rb

* Fix misc compile errors under VS2017

* Fix util/hashing under VS2017

std::hash<int> is not just a pass through in Microsoft's STL.

* Disable unsafe code warnings in VS2017

... where comparing against a reference implementation that has no easy
safe equivalent.

* Handle drive letters in paths on Windows