Upgrade C++14 to C++17 for Cocoapods#15083

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

cheryllin/c++17upgradepodsfirebase/firebase-ios-sdk:cheryllin/c++17upgradepodsCopy head branch name to clipboard