gofrontend: add missing break in Builtin_call_expression::do_flatten

    
    Reviewed-on: https://go-review.googlesource.com/29593

From-SVN: r240378

gcc/go/gofrontend/MERGE

-69668416034247ac6c7228c9dcbf6719af05b6ca
+34c4837efc21c35eb21f40efc9bb6b1d71dbda47
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -7066,6 +7066,7 @@ Builtin_call_expression::do_flatten(Gogo*, Named_object*,
 	      *pa = Expression::make_temporary_reference(temp, loc);
 	    }
 	}
+      break;
 
     case BUILTIN_LEN:
       Expression_list::iterator pa = this->args()->begin();