Tweak the union used for Extensions to support old generated code.
Tweak the union used for Extensions to support old generated code.

Support the old field name as well as the union to keep the old generated code
building.

Fixes #7555

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