Field-tracking is an experimental feature in the Go linker
where it statically tracks whether a Go struct field is used.
This CL modifies the generator to emit special markers that
tells the linker to know which fields to track.
Change-Id: I235da3f3d6c0ef2021b5fe1c16106ebb8fd6f557
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189558
Reviewed-by: Damien Neil <dneil@google.com>
