internal/fileinit: add FileDescriptor.ProtoLegacyRawDesc method

Add a ProtoLegacyRawDesc method for v1 registration support.

Change-Id: I4f03d022854c8d79da776610cb7d75ba9976334f
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/172241
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/fileinit/desc_init.go b/internal/fileinit/desc_init.go
index f339b0d..29d0531 100644
--- a/internal/fileinit/desc_init.go
+++ b/internal/fileinit/desc_init.go
@@ -12,10 +12,9 @@
 
 func newFileDesc(fb FileBuilder) *fileDesc {
 	file := &fileDesc{fileInit: fileInit{
-		RawDescriptor:     fb.RawDescriptor,
 		GoTypes:           fb.GoTypes,
 		DependencyIndexes: fb.DependencyIndexes,
-	}}
+	}, rawDesc: fb.RawDescriptor}
 	file.initDecls(len(fb.EnumOutputTypes), len(fb.MessageOutputTypes), len(fb.ExtensionOutputTypes))
 	file.unmarshalSeed(fb.RawDescriptor)