Upgrade c++14 to c++17 in cmake#15073

cherylEnkidu merged 2 commits intocheryllin/c++17upgradefirebase/firebase-ios-sdk:cheryllin/c++17upgradefrom

cheryllin/c++17upgradecmakefirebase/firebase-ios-sdk:cheryllin/c++17upgradecmakeCopy head branch name to clipboard