Code reorg to move all public headers under src/api#14371

wu-hui wants to merge 4 commits intocheryllin/firestoreSwiftCppfirebase/firebase-ios-sdk:cheryllin/firestoreSwiftCppfrom

wuandy/SwiftCppFixesfirebase/firebase-ios-sdk:wuandy/SwiftCppFixesCopy head branch name to clipboard