compiler: don't set btype_ too early for alias type

    
    CL 123362 introduced a bug in creating alias type's backend
    representation. A type's btype_ should not be set before
    named types are converted if it is a placeholder. For alias
    type, it is set too early. This may result in unresolved
    placeholders. This CL fixes it.
    
    Reviewed-on: https://go-review.googlesource.com/123975

From-SVN: r262833

gcc/go/gofrontend/MERGE

-d6338c94e5574b63469c740159d064e89c6718bf
+38850073f25f9de4f3daa33d799def3a33c942ab
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/types.cc

@@ -992,8 +992,10 @@ Type::get_backend(Gogo* gogo)
     return this->btype_;
 
   if (this->named_type() != NULL && this->named_type()->is_alias()) {
-    this->btype_ = this->unalias()->get_backend(gogo);
-    return this->btype_;
+    Btype* bt = this->unalias()->get_backend(gogo);
+    if (gogo != NULL && gogo->named_types_are_converted())
+      this->btype_ = bt;
+    return bt;
   }
 
   if (this->forward_declaration_type() != NULL