rename upb_Message_WhichOneof to upb_Message_WhichOneofByDef
and introduce upb_Message_WhichOneof that returns a minitable field without needing reflection
Except for when it's set to 0 for the immutable empty SmallSortedMap, but the number doesn't change anything for that immutable empty map (it's always empty, it will never hit the max).
This should save 4 bytes of memory per SmallSortedMap.
Future-proof edition 2023 against feature value support windows.
This "feature" hasn't been implemented yet, but this puts a placeholder down to prevent compatibility issues in future editions. Once we provide versioning support on individual feature values, we don't want them becoming usable from edition 2023 protos