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_lazy.go b/internal/fileinit/desc_lazy.go
index bbcc9d9..62ee81b 100644
--- a/internal/fileinit/desc_lazy.go
+++ b/internal/fileinit/desc_lazy.go
@@ -20,7 +20,7 @@
 
 func (file *fileDesc) lazyInit() *fileLazy {
 	file.once.Do(func() {
-		file.unmarshalFull(file.RawDescriptor)
+		file.unmarshalFull(file.rawDesc)
 		file.resolveImports()
 		file.resolveEnums()
 		file.resolveMessages()