reflect/protoregistry: add compile-time opt-out for registration conflicts
reflect/protoregistry: add compile-time opt-out for registration conflicts

The next release of this module will panic on registration conflicts.
While we have been warning that this will become a fatal error for about
a year now, it's almost certainly the case that some users still have
ambiguous registrations.

In addition to the GOLANG_PROTOBUF_REGISTRATION_CONFLICT environment variable,
also define a linker-initialized variable that users can set.

Change-Id: I3325d841b9b6c692e2862100d81aebdcfda069fc
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/295349
Trust: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Trust: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go