compiler/protogen: add (*GeneratedFile).Unskip
compiler/protogen: add (*GeneratedFile).Unskip

Simplifies some compiler plugins who want to avoid generating empty
files. With this API, generated files can be skipped by default, and
unskipped when actual code is generated.

Change-Id: I941c821646f5c4430a84e08bbad7e021434b1e71
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/232239
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go