Upgrade C++14 to C++17 for SPM#15072

cherylEnkidu wants to merge 1 commit intocheryllin/c++17upgradefirebase/firebase-ios-sdk:cheryllin/c++17upgradefrom

cheryllin/c++17upgradeSPMfirebase/firebase-ios-sdk:cheryllin/c++17upgradeSPMCopy head branch name to clipboard