[ObjC] Don't use unions and instead use memcpy for the type swaps. (#6672)
[ObjC] Don't use unions and instead use memcpy for the type swaps. (#6672)

The code in question hasn't change in a long time so the cause of
https://github.com/firebase/firebase-ios-sdk/issues/3851 still appears to be
an Xcode 11 clang change/bug; but this does appear to be slightly better
code for the work being done.

Cleanup along the way for #6679 

google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf