Explicitly handle Firestore/third_party/Immutable
Explicitly handle Firestore/third_party/Immutable

This allows Firestore/third_party to include a mix of Objective-C and
C++ code.

firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk,firebase/firebase-ios-sdk