internal/pragma: package to statically enforce certain properties
internal/pragma: package to statically enforce certain properties

Package pragma provides certain constructs that can be used to statically enforce
certain language properties.

Change-Id: I58a87bf7429f3be1b5aae418595fadda32f9d039
Reviewed-on: https://go-review.googlesource.com/127822
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Reviewed-by: Herbie Ong <86f376bf42b6daa37c768c730acd318ab6855de4@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go