all: implement proto1 weak fields

This implements generation of and reflection support for weak fields.
Weak fields are a proto1 feature where the "weak" option can be specified
on a singular message field. A weak reference results in generated code
that does not directly link in the dependency containing the weak message.

Weak field support is not added to any of the serialization logic.

Change-Id: I08ccfa72bc80b2ffb6af527a1677a0a81dcf33fb
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185399
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/reflect/protodesc/file_test.go b/reflect/protodesc/file_test.go
index f3e0b0a..b20be3f 100644
--- a/reflect/protodesc/file_test.go
+++ b/reflect/protodesc/file_test.go
@@ -10,6 +10,7 @@
 	"testing"
 
 	"google.golang.org/protobuf/encoding/prototext"
+	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/reflect/protoregistry"
 
@@ -767,7 +768,13 @@
 				options: {message_set_wire_format:true}
 			}]}]
 		`),
-		wantErr: `message "M.M" is an invalid proto1 MessageSet`,
+		wantErr: func() string {
+			if flags.Proto1Legacy {
+				return `message "M.M" is an invalid proto1 MessageSet`
+			} else {
+				return `message "M.M" is a MessageSet, which is a legacy proto1 feature that is no longer supported`
+			}
+		}(),
 	}, {
 		label: "valid MessageSet",
 		inDesc: mustParseFile(`
@@ -779,6 +786,13 @@
 				options:         {message_set_wire_format:true}
 			}]}]
 		`),
+		wantErr: func() string {
+			if flags.Proto1Legacy {
+				return ""
+			} else {
+				return `message "M.M" is a MessageSet, which is a legacy proto1 feature that is no longer supported`
+			}
+		}(),
 	}, {
 		label: "invalid extension ranges in proto3",
 		inDesc: mustParseFile(`