[Serialization] Make sure to emit an XREF before XREF_OPAQUE_RETURN_TYPE_PATH_PIECE

We might have an opaque return type path piece after a generic type
param type, so make sure to emit the XREF entry before the opaque return
type path piece as well.

Fixes rdar://53958358

Author: Harlan Haskins

lib/Serialization/Serialization.cpp

@@ -1731,6 +1731,8 @@ void Serializer::writeCrossReference(const DeclContext *DC, uint32_t pathLen) {
   case DeclContextKind::GenericTypeDecl: {
     auto generic = cast<GenericTypeDecl>(DC);
 
+    writeCrossReference(DC->getParent(), pathLen + 1);
+
     // Opaque return types are unnamed and need a special xref.
     if (auto opaque = dyn_cast<OpaqueTypeDecl>(generic)) {
       if (!opaque->hasName()) {
@@ -1744,8 +1746,6 @@ void Serializer::writeCrossReference(const DeclContext *DC, uint32_t pathLen) {
     }
 
     assert(generic->hasName());
-    
-    writeCrossReference(DC->getParent(), pathLen + 1);
 
     abbrCode = DeclTypeAbbrCodes[XRefTypePathPieceLayout::Code];