compiler: use the underlying type to build placeholder type for alias

When asking for a placeholder type of an alias type, build a
    placeholder for the underlying type, instead of treating the
    alias as a named type and calling get_backend. The latter may
    fail as we may not be ready to build a complete backend type. We
    have already used a unified backend type for alias type and its
    underlying type. Do the same for placeholders as well.
    
    Reviewed-on: https://go-review.googlesource.com/138635

From-SVN: r264773

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-2f56d51c6b3104242613c74b02fa6c63a2fe16c5
+53d0d7ca278a5612fcdb5fb098e7bf950a0178ef
 
 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

@@ -1125,6 +1125,8 @@ Type::get_backend_placeholder(Gogo* gogo)
     case TYPE_FORWARD:
       // Named types keep track of their own dependencies and manage
       // their own placeholders.
+      if (this->named_type() != NULL && this->named_type()->is_alias())
+        return this->unalias()->get_backend_placeholder(gogo);
       return this->get_backend(gogo);
 
     case TYPE_INTERFACE: